The whalesay image could be improved. It would be nice if you didn't have to think of something to say. And you type a lot to get whalesay to talk.
docker run docker/whalesay cowsay boo-boo
In this next section, you will improve the whalesay image by building a new version that “talks on its own” and requires fewer words to run.
In this step, you use your favorite text editor to write a short Dockerfile. A Dockerfile describes the software that is “baked” into an image. It isn't just ingredients tho, it can tell the software what environment to use or what commands to run. Your recipe is going to be very short.
Go back to your command terminal window.
Make a new directory by typing mkdir mydockerbuild and pressing RETURN.
$ mkdir mydockerbuild
This directory serves as the “context” for your build. The context just means it contains all the things you need to build your image.
Change to your new directory.
$ cd mydockerbuild
Right now the directory is empty.
Create a Dockerfile in the directory by typing touch Dockerfile and pressing RETURN.
$ touch Dockerfile
The command appears to do nothing but it actually creates the Dockerfile in the current directory. Just type ls Dockerfile to see it.
$ ls Dockerfile Dockerfile
Open the Dockerfile in a visual text editor like Atom or Sublime, or a text based editor like vi, or nano (https://www.nano-editor.org/).
Add a line to the file like this:
FROM docker/whalesay:latest
The FROM keyword tells Docker which image your image is based on. Whalesay is cute and has the cowsay program already, so we'll start there.
Now, add the fortunes program to the image.
RUN apt-get -y update && apt-get install -y fortunes
The fortunes program has a command that prints out wise sayings for our whale to say. So, the first step is to install it. This line installs the software into the image.
Once the image has the software it needs, you instruct the software to run when the image is loaded.
CMD /usr/games/fortune -a | cowsay
This line tells the fortune program to pass a nifty quote to the cowsay program.
Check your work, your file should look like this:
FROM docker/whalesay:latest RUN apt-get -y update && apt-get install -y fortunes CMD /usr/games/fortune -a | cowsay
Save and close your Dockerfile.
At this point, you have all your software ingredients and behaviors described in a Dockerfile. You are ready to build a new image.
At the command line, make sure the Dockerfile is in the current directory by typing cat Dockerfile
$ cat Dockerfile FROM docker/whalesay:latest RUN apt-get -y update && apt-get install -y fortunes CMD /usr/games/fortune -a | cowsay
Now, build your new image by typing the docker build -t docker-whale . command in your terminal (don't forget the . period).
$ docker build -t docker-whale . Sending build context to Docker daemon 158.8 MB ...snip... Removing intermediate container a8e6faa88df3 Successfully built 7d9495d03763
The command takes several seconds to run and reports its outcome. Before you do anything with the new image, take a minute to learn about the Dockerfile build process.
The docker build -t docker-whale . command takes the Dockerfile in the current directory, and builds an image called docker-whale on your local machine. The command takes about a minute and its output looks really long and complex. In this section, you learn what each message means.
First Docker checks to make sure it has everything it needs to build.
Sending build context to Docker daemon 158.8 MB
Then, Docker loads with the whalesay image. It already has this image locally as you might recall from the last page. So, Docker doesn't need to download it.
Step 0 : FROM docker/whalesay:latest ---> fb434121fc77
Docker moves onto the next step which is to update the apt-get package manager. This takes a lot of lines, no need to list them all again here.
Step 1 : RUN apt-get -y update && apt-get install -y fortunes ---> Running in 27d224dfa5b2 Ign http://archive.ubuntu.com trusty InRelease Ign http://archive.ubuntu.com trusty-updates InRelease Ign http://archive.ubuntu.com trusty-security InRelease Hit http://archive.ubuntu.com trusty Release.gpg ....snip... Get:15 http://archive.ubuntu.com trusty-security/restricted amd64 Packages [14.8 kB] Get:16 http://archive.ubuntu.com trusty-security/universe amd64 Packages [134 kB] Reading package lists... ---> eb06e47a01d2
Then, Docker installs the new fortunes software.
Removing intermediate container e2a84b5f390f Step 2 : RUN apt-get install -y fortunes ---> Running in 23aa52c1897c Reading package lists... Building dependency tree... Reading state information... The following extra packages will be installed: fortune-mod fortunes-min librecode0 Suggested packages: x11-utils bsdmainutils The following NEW packages will be installed: fortune-mod fortunes fortunes-min librecode0 0 upgraded, 4 newly installed, 0 to remove and 3 not upgraded. Need to get 1961 kB of archives. After this operation, 4817 kB of additional disk space will be used. Get:1 http://archive.ubuntu.com/ubuntu/ trusty/main librecode0 amd64 3.6-21 [771 kB] ...snip...... Setting up fortunes (1:1.99.1-7) ... Processing triggers for libc-bin (2.19-0ubuntu6.6) ... ---> c81071adeeb5 Removing intermediate container 23aa52c1897c
Finally, Docker finishes the build and reports its outcome.
Step 3 : CMD /usr/games/fortune -a | cowsay ---> Running in a8e6faa88df3 ---> 7d9495d03763 Removing intermediate container a8e6faa88df3 Successfully built 7d9495d03763
In this step, you verify the new images is on your computer and then you run your new image.
Open a command line terminal.
Type docker images and press RETURN.
This command, you might remember, lists the images you have locally.
$ docker images REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE docker-whale latest 7d9495d03763 4 minutes ago 273.7 MB docker/whalesay latest fb434121fc77 4 hours ago 247 MB hello-world latest 91c95931e552 5 weeks ago 910 B
Run your new image by typing docker run docker-whale and pressing RETURN.
$ docker run docker-whale
_________________________________________
/ "He was a modest, good-humored boy. It \
\ was Oxford that made him insufferable." /
-----------------------------------------
\
\
\
## .
## ## ## ==
## ## ## ## ===
/""""""""""""""""___/ ===
~~~ {~~ ~~~~ ~~~ ~~~~ ~~ ~ / ===- ~~~
\______ o __/
\ \ __/
\____\______/
As you can see, you‘ve made the whale a lot smarter. It finds its own things to say and the command line is a lot shorter! You may also notice that Docker didn’t have to download anything. That is because the image was built locally and is already available.
On this page, you learned to build an image by writing your own Dockerfile. You ran your image in a container. You also just used Linux from your Mac yet again. In the next section, you take the first step in sharing your image by creating a Docker Hub account.