Get started with images
In the introduction we‘ve discovered that Docker images are the basis of containers. In the previous sections we’ve used Docker images that already exist, for example the ubuntu image and the training/webapp image.
We‘ve also discovered that Docker stores downloaded images on the Docker host. If an image isn’t already present on the host then it'll be downloaded from a registry: by default the Docker Hub Registry.
In this section we're going to explore Docker images a bit more including:
- Managing and working with images locally on your Docker host;
- Creating basic images;
- Uploading images to Docker Hub Registry.
Listing images on the host
Let's start with listing the images we have locally on our host. You can do this using the docker images command like so:
$ docker images REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE training/webapp latest fc77f57ad303 3 weeks ago 280.5 MB ubuntu 13.10 5e019ab7bf6d 4 weeks ago 180 MB ubuntu saucy 5e019ab7bf6d 4 weeks ago 180 MB ubuntu 12.04 74fe38d11401 4 weeks ago 209.6 MB ubuntu precise 74fe38d11401 4 weeks ago 209.6 MB ubuntu 12.10 a7cf8ae4e998 4 weeks ago 171.3 MB ubuntu quantal a7cf8ae4e998 4 weeks ago 171.3 MB ubuntu 14.04 99ec81b80c55 4 weeks ago 266 MB ubuntu latest 99ec81b80c55 4 weeks ago 266 MB ubuntu trusty 99ec81b80c55 4 weeks ago 266 MB ubuntu 13.04 316b678ddf48 4 weeks ago 169.4 MB ubuntu raring 316b678ddf48 4 weeks ago 169.4 MB ubuntu 10.04 3db9c44f4520 4 weeks ago 183 MB ubuntu lucid 3db9c44f4520 4 weeks ago 183 MB
We can see the images we've previously used in our user guide. Each has been downloaded from Docker Hub when we launched a container using that image.
We can see three crucial pieces of information about our images in the listing.
- What repository they came from, for example
ubuntu. - The tags for each image, for example
14.04. - The image ID of each image.
Note: Previously, the
docker imagescommand supported the--treeand--dotarguments, which displayed different visualizations of the image data. Docker core removed this functionality in the 1.7 version. If you liked this functionality, you can still find it in the third-party dockviz tool.
A repository potentially holds multiple variants of an image. In the case of our ubuntu image we can see multiple variants covering Ubuntu 10.04, 12.04, 12.10, 13.04, 13.10 and 14.04. Each variant is identified by a tag and you can refer to a tagged image like so:
ubuntu:14.04
So when we run a container we refer to a tagged image like so:
$ docker run -t -i ubuntu:14.04 /bin/bash
If instead we wanted to run an Ubuntu 12.04 image we'd use:
$ docker run -t -i ubuntu:12.04 /bin/bash
If you don't specify a variant, for example you just use ubuntu, then Docker will default to using the ubuntu:latest image.
Tip: We recommend you always use a specific tagged image, for example
ubuntu:12.04. That way you always know exactly what variant of an image is being used.
Getting a new image
So how do we get new images? Well Docker will automatically download any image we use that isn‘t already present on the Docker host. But this can potentially add some time to the launch of a container. If we want to pre-load an image we can download it using the docker pull command. Let’s say we'd like to download the centos image.
$ docker pull centos Pulling repository centos b7de3133ff98: Pulling dependent layers 5cc9e91966f7: Pulling fs layer 511136ea3c5a: Download complete ef52fb1fe610: Download complete . . . Status: Downloaded newer image for centos
We can see that each layer of the image has been pulled down and now we can run a container from this image and we won't have to wait to download the image.
$ docker run -t -i centos /bin/bash bash-4.1#
Finding images
One of the features of Docker is that a lot of people have created Docker images for a variety of purposes. Many of these have been uploaded to Docker Hub. We can search these images on the Docker Hub website.
We can also search for images on the command line using the docker search command. Let's say our team wants an image with Ruby and Sinatra installed on which to do our web application development. We can search for a suitable image by using the docker search command to find all the images that contain the term sinatra.
$ docker search sinatra NAME DESCRIPTION STARS OFFICIAL AUTOMATED training/sinatra Sinatra training image 0 [OK] marceldegraaf/sinatra Sinatra test app 0 mattwarren/docker-sinatra-demo 0 [OK] luisbebop/docker-sinatra-hello-world 0 [OK] bmorearty/handson-sinatra handson-ruby + Sinatra for Hands on with D... 0 subwiz/sinatra 0 bmorearty/sinatra 0 . . .
We can see we‘ve returned a lot of images that use the term sinatra. We’ve returned a list of image names, descriptions, Stars (which measure the social popularity of images - if a user likes an image then they can “star” it), and the Official and Automated build statuses. Official Repositories are a carefully curated set of Docker repositories supported by Docker, Inc. Automated repositories are Automated Builds that allow you to validate the source and content of an image.
We‘ve reviewed the images available to use and we decided to use the training/sinatra image. So far we’ve seen two types of images repositories, images like ubuntu, which are called base or root images. These base images are provided by Docker Inc and are built, validated and supported. These can be identified by their single word names.
We‘ve also seen user images, for example the training/sinatra image we’ve chosen. A user image belongs to a member of the Docker community and is built and maintained by them. You can identify user images as they are always prefixed with the user name, here training, of the user that created them.
Pulling our image
We've identified a suitable image, training/sinatra, and now we can download it using the docker pull command.
$ docker pull training/sinatra
The team can now use this image by running their own containers.
$ docker run -t -i training/sinatra /bin/bash root@a8cb6ce02d85:/#
Creating our own images
The team has found the training/sinatra image pretty useful but it's not quite what they need and we need to make some changes to it. There are two ways we can update and create images.
- We can update a container created from an image and commit the results to an image.
- We can use a
Dockerfileto specify instructions to create an image.
Updating and committing an image
To update an image we first need to create a container from the image we'd like to update.
$ docker run -t -i training/sinatra /bin/bash root@0b2616b0e5a8:/#
Note: Take note of the container ID that has been created,
0b2616b0e5a8, as we'll need it in a moment.
Inside our running container let's add the json gem.
root@0b2616b0e5a8:/# gem install json
Once this has completed let's exit our container using the exit command.
Now we have a container with the change we want to make. We can then commit a copy of this container to an image using the docker commit command.
$ docker commit -m "Added json gem" -a "Kate Smith" \ 0b2616b0e5a8 ouruser/sinatra:v2 4f177bd27a9ff0f6dc2a830403925b5360bfe0b93d476f7fc3231110e7f71b1c
Here we‘ve used the docker commit command. We’ve specified two flags: -m and -a. The -m flag allows us to specify a commit message, much like you would with a commit on a version control system. The -a flag allows us to specify an author for our update.
We‘ve also specified the container we want to create this new image from, 0b2616b0e5a8 (the ID we recorded earlier) and we’ve specified a target for the image:
ouruser/sinatra:v2
Let‘s break this target down. It consists of a new user, ouruser, that we’re writing this image to. We‘ve also specified the name of the image, here we’re keeping the original image name sinatra. Finally we're specifying a tag for the image: v2.
We can then look at our new ouruser/sinatra image using the docker images command.
$ docker images REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE training/sinatra latest 5bc342fa0b91 10 hours ago 446.7 MB ouruser/sinatra v2 3c59e02ddd1a 10 hours ago 446.7 MB ouruser/sinatra latest 5db5f8471261 10 hours ago 446.7 MB
To use our new image to create a container we can then:
$ docker run -t -i ouruser/sinatra:v2 /bin/bash root@78e82f680994:/#
Building an image from a Dockerfile
Using the docker commit command is a pretty simple way of extending an image but it‘s a bit cumbersome and it’s not easy to share a development process for images amongst a team. Instead we can use a new command, docker build, to build new images from scratch.
To do this we create a Dockerfile that contains a set of instructions that tell Docker how to build our image.
Let's create a directory and a Dockerfile first.
$ mkdir sinatra $ cd sinatra $ touch Dockerfile
If you are using Docker Machine on Windows, you may access your host directory by cd to /c/Users/your_user_name.
Each instruction creates a new layer of the image. Let's look at a simple example now for building our own Sinatra image for our development team.
# This is a comment FROM ubuntu:14.04 MAINTAINER Kate Smith <ksmith@example.com> RUN apt-get update && apt-get install -y ruby ruby-dev RUN gem install sinatra
Let's look at what our Dockerfile does. Each instruction prefixes a statement and is capitalized.
INSTRUCTION statement
Note: We use
#to indicate a comment
The first instruction FROM tells Docker what the source of our image is, in this case we're basing our new image on an Ubuntu 14.04 image.
Next we use the MAINTAINER instruction to specify who maintains our new image.
Lastly, we‘ve specified two RUN instructions. A RUN instruction executes a command inside the image, for example installing a package. Here we’re updating our APT cache, installing Ruby and RubyGems and then installing the Sinatra gem.
Note: There are a lot more instructions available to us in a Dockerfile.
Now let's take our Dockerfile and use the docker build command to build an image.
$ docker build -t ouruser/sinatra:v2 . Sending build context to Docker daemon 2.048 kB Sending build context to Docker daemon Step 0 : FROM ubuntu:14.04 ---> e54ca5efa2e9 Step 1 : MAINTAINER Kate Smith <ksmith@example.com> ---> Using cache ---> 851baf55332b Step 2 : RUN apt-get update && apt-get install -y ruby ruby-dev ---> Running in 3a2558904e9b Selecting previously unselected package libasan0:amd64. (Reading database ... 11518 files and directories currently installed.) Preparing to unpack .../libasan0_4.8.2-19ubuntu1_amd64.deb ... Unpacking libasan0:amd64 (4.8.2-19ubuntu1) ... Selecting previously unselected package libatomic1:amd64. Preparing to unpack .../libatomic1_4.8.2-19ubuntu1_amd64.deb ... Unpacking libatomic1:amd64 (4.8.2-19ubuntu1) ... Selecting previously unselected package libgmp10:amd64. Preparing to unpack .../libgmp10_2%3a5.1.3+dfsg-1ubuntu1_amd64.deb ... Unpacking libgmp10:amd64 (2:5.1.3+dfsg-1ubuntu1) ... Selecting previously unselected package libisl10:amd64. Preparing to unpack .../libisl10_0.12.2-1_amd64.deb ... Unpacking libisl10:amd64 (0.12.2-1) ... Selecting previously unselected package libcloog-isl4:amd64. Preparing to unpack .../libcloog-isl4_0.18.2-1_amd64.deb ... Unpacking libcloog-isl4:amd64 (0.18.2-1) ... Selecting previously unselected package libgomp1:amd64. Preparing to unpack .../libgomp1_4.8.2-19ubuntu1_amd64.deb ... Unpacking libgomp1:amd64 (4.8.2-19ubuntu1) ... Selecting previously unselected package libitm1:amd64. Preparing to unpack .../libitm1_4.8.2-19ubuntu1_amd64.deb ... Unpacking libitm1:amd64 (4.8.2-19ubuntu1) ... Selecting previously unselected package libmpfr4:amd64. Preparing to unpack .../libmpfr4_3.1.2-1_amd64.deb ... Unpacking libmpfr4:amd64 (3.1.2-1) ... Selecting previously unselected package libquadmath0:amd64. Preparing to unpack .../libquadmath0_4.8.2-19ubuntu1_amd64.deb ... Unpacking libquadmath0:amd64 (4.8.2-19ubuntu1) ... Selecting previously unselected package libtsan0:amd64. Preparing to unpack .../libtsan0_4.8.2-19ubuntu1_amd64.deb ... Unpacking libtsan0:amd64 (4.8.2-19ubuntu1) ... Selecting previously unselected package libyaml-0-2:amd64. Preparing to unpack .../libyaml-0-2_0.1.4-3ubuntu3_amd64.deb ... Unpacking libyaml-0-2:amd64 (0.1.4-3ubuntu3) ... Selecting previously unselected package libmpc3:amd64. Preparing to unpack .../libmpc3_1.0.1-1ubuntu1_amd64.deb ... Unpacking libmpc3:amd64 (1.0.1-1ubuntu1) ... Selecting previously unselected package openssl. Preparing to unpack .../openssl_1.0.1f-1ubuntu2.4_amd64.deb ... Unpacking openssl (1.0.1f-1ubuntu2.4) ... Selecting previously unselected package ca-certificates. Preparing to unpack .../ca-certificates_20130906ubuntu2_all.deb ... Unpacking ca-certificates (20130906ubuntu2) ... Selecting previously unselected package manpages. Preparing to unpack .../manpages_3.54-1ubuntu1_all.deb ... Unpacking manpages (3.54-1ubuntu1) ... Selecting previously unselected package binutils. Preparing to unpack .../binutils_2.24-5ubuntu3_amd64.deb ... Unpacking binutils (2.24-5ubuntu3) ... Selecting previously unselected package cpp-4.8. Preparing to unpack .../cpp-4.8_4.8.2-19ubuntu1_amd64.deb ... Unpacking cpp-4.8 (4.8.2-19ubuntu1) ... Selecting previously unselected package cpp. Preparing to unpack .../cpp_4%3a4.8.2-1ubuntu6_amd64.deb ... Unpacking cpp (4:4.8.2-1ubuntu6) ... Selecting previously unselected package libgcc-4.8-dev:amd64. Preparing to unpack .../libgcc-4.8-dev_4.8.2-19ubuntu1_amd64.deb ... Unpacking libgcc-4.8-dev:amd64 (4.8.2-19ubuntu1) ... Selecting previously unselected package gcc-4.8. Preparing to unpack .../gcc-4.8_4.8.2-19ubuntu1_amd64.deb ... Unpacking gcc-4.8 (4.8.2-19ubuntu1) ... Selecting previously unselected package gcc. Preparing to unpack .../gcc_4%3a4.8.2-1ubuntu6_amd64.deb ... Unpacking gcc (4:4.8.2-1ubuntu6) ... Selecting previously unselected package libc-dev-bin. Preparing to unpack .../libc-dev-bin_2.19-0ubuntu6_amd64.deb ... Unpacking libc-dev-bin (2.19-0ubuntu6) ... Selecting previously unselected package linux-libc-dev:amd64. Preparing to unpack .../linux-libc-dev_3.13.0-30.55_amd64.deb ... Unpacking linux-libc-dev:amd64 (3.13.0-30.55) ... Selecting previously unselected package libc6-dev:amd64. Preparing to unpack .../libc6-dev_2.19-0ubuntu6_amd64.deb ... Unpacking libc6-dev:amd64 (2.19-0ubuntu6) ... Selecting previously unselected package ruby. Preparing to unpack .../ruby_1%3a1.9.3.4_all.deb ... Unpacking ruby (1:1.9.3.4) ... Selecting previously unselected package ruby1.9.1. Preparing to unpack .../ruby1.9.1_1.9.3.484-2ubuntu1_amd64.deb ... Unpacking ruby1.9.1 (1.9.3.484-2ubuntu1) ... Selecting previously unselected package libruby1.9.1. Preparing to unpack .../libruby1.9.1_1.9.3.484-2ubuntu1_amd64.deb ... Unpacking libruby1.9.1 (1.9.3.484-2ubuntu1) ... Selecting previously unselected package manpages-dev. Preparing to unpack .../manpages-dev_3.54-1ubuntu1_all.deb ... Unpacking manpages-dev (3.54-1ubuntu1) ... Selecting previously unselected package ruby1.9.1-dev. Preparing to unpack .../ruby1.9.1-dev_1.9.3.484-2ubuntu1_amd64.deb ... Unpacking ruby1.9.1-dev (1.9.3.484-2ubuntu1) ... Selecting previously unselected package ruby-dev. Preparing to unpack .../ruby-dev_1%3a1.9.3.4_all.deb ... Unpacking ruby-dev (1:1.9.3.4) ... Setting up libasan0:amd64 (4.8.2-19ubuntu1) ... Setting up libatomic1:amd64 (4.8.2-19ubuntu1) ... Setting up libgmp10:amd64 (2:5.1.3+dfsg-1ubuntu1) ... Setting up libisl10:amd64 (0.12.2-1) ... Setting up libcloog-isl4:amd64 (0.18.2-1) ... Setting up libgomp1:amd64 (4.8.2-19ubuntu1) ... Setting up libitm1:amd64 (4.8.2-19ubuntu1) ... Setting up libmpfr4:amd64 (3.1.2-1) ... Setting up libquadmath0:amd64 (4.8.2-19ubuntu1) ... Setting up libtsan0:amd64 (4.8.2-19ubuntu1) ... Setting up libyaml-0-2:amd64 (0.1.4-3ubuntu3) ... Setting up libmpc3:amd64 (1.0.1-1ubuntu1) ... Setting up openssl (1.0.1f-1ubuntu2.4) ... Setting up ca-certificates (20130906ubuntu2) ... debconf: unable to initialize frontend: Dialog debconf: (TERM is not set, so the dialog frontend is not usable.) debconf: falling back to frontend: Readline debconf: unable to initialize frontend: Readline debconf: (This frontend requires a controlling tty.) debconf: falling back to frontend: Teletype Setting up manpages (3.54-1ubuntu1) ... Setting up binutils (2.24-5ubuntu3) ... Setting up cpp-4.8 (4.8.2-19ubuntu1) ... Setting up cpp (4:4.8.2-1ubuntu6) ... Setting up libgcc-4.8-dev:amd64 (4.8.2-19ubuntu1) ... Setting up gcc-4.8 (4.8.2-19ubuntu1) ... Setting up gcc (4:4.8.2-1ubuntu6) ... Setting up libc-dev-bin (2.19-0ubuntu6) ... Setting up linux-libc-dev:amd64 (3.13.0-30.55) ... Setting up libc6-dev:amd64 (2.19-0ubuntu6) ... Setting up manpages-dev (3.54-1ubuntu1) ... Setting up libruby1.9.1 (1.9.3.484-2ubuntu1) ... Setting up ruby1.9.1-dev (1.9.3.484-2ubuntu1) ... Setting up ruby-dev (1:1.9.3.4) ... Setting up ruby (1:1.9.3.4) ... Setting up ruby1.9.1 (1.9.3.484-2ubuntu1) ... Processing triggers for libc-bin (2.19-0ubuntu6) ... Processing triggers for ca-certificates (20130906ubuntu2) ... Updating certificates in /etc/ssl/certs... 164 added, 0 removed; done. Running hooks in /etc/ca-certificates/update.d....done. ---> c55c31703134 Removing intermediate container 3a2558904e9b Step 3 : RUN gem install sinatra ---> Running in 6b81cb6313e5 unable to convert "\xC3" to UTF-8 in conversion from ASCII-8BIT to UTF-8 to US-ASCII for README.rdoc, skipping unable to convert "\xC3" to UTF-8 in conversion from ASCII-8BIT to UTF-8 to US-ASCII for README.rdoc, skipping Successfully installed rack-1.5.2 Successfully installed tilt-1.4.1 Successfully installed rack-protection-1.5.3 Successfully installed sinatra-1.4.5 4 gems installed Installing ri documentation for rack-1.5.2... Installing ri documentation for tilt-1.4.1... Installing ri documentation for rack-protection-1.5.3... Installing ri documentation for sinatra-1.4.5... Installing RDoc documentation for rack-1.5.2... Installing RDoc documentation for tilt-1.4.1... Installing RDoc documentation for rack-protection-1.5.3... Installing RDoc documentation for sinatra-1.4.5... ---> 97feabe5d2ed Removing intermediate container 6b81cb6313e5 Successfully built 97feabe5d2ed
We've specified our docker build command and used the -t flag to identify our new image as belonging to the user ouruser, the repository name sinatra and given it the tag v2.
We've also specified the location of our Dockerfile using the . to indicate a Dockerfile in the current directory.
Note: You can also specify a path to a
Dockerfile.
Now we can see the build process at work. The first thing Docker does is upload the build context: basically the contents of the directory you're building in. This is done because the Docker daemon does the actual build of the image and it needs the local context to do it.
Next we can see each instruction in the Dockerfile being executed step-by-step. We can see that each step creates a new container, runs the instruction inside that container and then commits that change - just like the docker commit work flow we saw earlier. When all the instructions have executed we're left with the 97feabe5d2ed image (also helpfully tagged as ouruser/sinatra:v2) and all intermediate containers will get removed to clean things up.
Note: An image can't have more than 127 layers regardless of the storage driver. This limitation is set globally to encourage optimization of the overall size of images.
We can then create a container from our new image.
$ docker run -t -i ouruser/sinatra:v2 /bin/bash root@8196968dac35:/#
Note: This is just a brief introduction to creating images. We‘ve skipped a whole bunch of other instructions that you can use. We’ll see more of those instructions in later sections of the Guide or you can refer to the
Dockerfilereference for a detailed description and examples of every instruction. To help you write a clear, readable, maintainableDockerfile, we've also written aDockerfileBest Practices guide.
More
To learn more, check out the Dockerfile tutorial.
Setting tags on an image
You can also add a tag to an existing image after you commit or build it. We can do this using the docker tag command. Let's add a new tag to our ouruser/sinatra image.
$ docker tag 5db5f8471261 ouruser/sinatra:devel
The docker tag command takes the ID of the image, here 5db5f8471261, and our user name, the repository name and the new tag.
Let's see our new tag using the docker images command.
$ docker images ouruser/sinatra REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE ouruser/sinatra latest 5db5f8471261 11 hours ago 446.7 MB ouruser/sinatra devel 5db5f8471261 11 hours ago 446.7 MB ouruser/sinatra v2 5db5f8471261 11 hours ago 446.7 MB
Image Digests
Images that use the v2 or later format have a content-addressable identifier called a digest. As long as the input used to generate the image is unchanged, the digest value is predictable. To list image digest values, use the --digests flag:
$ docker images --digests | head REPOSITORY TAG DIGEST IMAGE ID CREATED VIRTUAL SIZE ouruser/sinatra latest sha256:cbbf2f9a99b47fc460d422812b6a5adff7dfee951d8fa2e4a98caa0382cfbdbf 5db5f8471261 11 hours ago 446.7 MB
When pushing or pulling to a 2.0 registry, the push or pull command output includes the image digest. You can pull using a digest value.
$ docker pull ouruser/sinatra@cbbf2f9a99b47fc460d422812b6a5adff7dfee951d8fa2e4a98caa0382cfbdbf
You can also reference by digest in create, run, and rmi commands, as well as the FROM image reference in a Dockerfile.
Push an image to Docker Hub
Once you've built or created a new image you can push it to Docker Hub using the docker push command. This allows you to share it with others, either publicly, or push it into a private repository.
$ docker push ouruser/sinatra The push refers to a repository [ouruser/sinatra] (len: 1) Sending image list Pushing repository ouruser/sinatra (3 tags) . . .
Remove an image from the host
You can also remove images on your Docker host in a way similar to containers using the docker rmi command.
Let‘s delete the training/sinatra image as we don’t need it anymore.
$ docker rmi training/sinatra Untagged: training/sinatra:latest Deleted: 5bc342fa0b91cabf65246837015197eecfa24b2213ed6a51a8974ae250fedd8d Deleted: ed0fffdcdae5eb2c3a55549857a8be7fc8bc4241fb19ad714364cbfd7a56b22f Deleted: 5c58979d73ae448df5af1d8142436d81116187a7633082650549c52c3a2418f0
Note: In order to remove an image from the host, please make sure that there are no containers actively based on it.
Next steps
Until now we've seen how to build individual applications inside Docker containers. Now learn how to build whole application stacks with Docker by linking together multiple Docker containers.
Test your Dockerfile knowledge with the Dockerfile tutorial.
Go to Linking Containers Together.