Work with a development container

In this section, you learn to develop like a member of Docker‘s core team. The docker repository includes a Dockerfile at its root. This file defines Docker’s development environment. The Dockerfile lists the environment's dependencies: system libraries and binaries, Go environment, Go dependencies, etc.

Docker's development environment is itself, ultimately a Docker container. You use the docker repository and its Dockerfile to create a Docker image, run a Docker container, and develop code in the container. Docker itself builds, tests, and releases new Docker versions using this container.

If you followed the procedures that set up Git for contributing, you should have a fork of the docker/docker repository. You also created a branch called dry-run-test. In this section, you continue working with your fork on this branch.

Clean your host of Docker artifacts

Docker developers run the latest stable release of the Docker software (with Boot2Docker if their machine is Mac OS X). They clean their local hosts of unnecessary Docker artifacts such as stopped containers or unused images. Cleaning unnecessary artifacts isn't strictly necessary, but it is good practice, so it is included here.

To remove unnecessary artifacts,

Verify that you have no unnecessary containers running on your host.
```
 $ docker ps
```
You should see something similar to the following:
There are no running containers on this host. If you have running but unused containers, stop and then remove them with the docker stop and docker rm commands.
Verify that your host has no dangling images.
```
 $ docker images
```
You should see something similar to the following:
This host has no images. You may have one or more dangling images. A dangling image is not used by a running container and is not an ancestor of another image on your system. A fast way to remove dangling containers is the following:
```
 $ docker rmi -f $(docker images -q -a -f dangling=true)
```
This command uses docker images to list all images (-a flag) by numeric IDs (-q flag) and filter them to find dangling images (-f dangling=true). Then, the docker rmi command forcibly (-f flag) removes the resulting list. To remove just one image, use the docker rmi ID command.

Build an image

If you followed the last procedure, your host is clean of unnecessary images and containers. In this section, you build an image from the Docker development environment.

Open a terminal.
Mac users, use boot2docker status to make sure Boot2Docker is running. You may need to run eval "$(boot2docker shellinit)" to initialize your shell environment.
Change into the root of your forked repository.
```
 $ cd ~/repos/docker-fork 
```
If you are following along with this guide, you created a dry-run-test branch when you set up Git for contributing.
Ensure you are on your dry-run-test branch.
```
 $ git checkout dry-run-test
```
If you get a message that the branch doesn't exist, add the -b flag (git checkout -b dry-run-test) so the command both creates the branch and checks it out.
Compile your development environment container into an image.
```
 $ docker build -t dry-run-test .
```
The docker build command returns informational message as it runs. The first build may take a few minutes to create an image. Using the instructions in the Dockerfile, the build may need to download source and other images. A successful build returns a final status message similar to the following:
```
 Successfully built 676815d59283
```
List your Docker images again.
```
 $ docker images
```
You should see something similar to this:
Locate your new dry-run-test image in the list. You should also see a number of ubuntu images. The build process creates these. They are the ancestors of your new Docker development image. When you next rebuild your image, the build process reuses these ancestors images if they exist.
Keeping the ancestor images improves the build performance. When you rebuild the child image, the build process uses the local ancestors rather than retrieving them from the Hub. The build process gets new ancestors only if DockerHub has updated versions.

Start a container and run a test

At this point, you have created a new Docker development environment image. Now, you‘ll use this image to create a Docker container to develop in. Then, you’ll build and run a docker binary in your container.

Open two additional terminals on your host.
At this point, you'll have about three terminals open.
Mac OS X users, make sure you run eval "$(boot2docker shellinit)" in any new terminals.
In a terminal, create a new container from your dry-run-test image.
```
 $ docker run --privileged --rm -ti dry-run-test /bin/bash
 root@5f8630b873fe:/go/src/github.com/docker/docker# 
```
The command creates a container from your dry-run-test image. It opens an interactive terminal (-ti) running a /bin/bash shell. The --privileged flag gives the container access to kernel features and device access. This flag allows you to run a container in a container. Finally, the -rm flag instructs Docker to remove the container when you exit the /bin/bash shell.
The container includes the source of your image repository in the /go/src/github.com/docker/docker directory. Try listing the contents to verify they are the same as that of your docker-fork repo.
Investigate your container bit.
If you do a go version you'll find the go language is part of the container.
```
 root@31ed86e9ddcf:/go/src/github.com/docker/docker# go version
 go version go1.4.2 linux/amd64
```
Similarly, if you do a docker version you find the container has no docker binary.
```
 root@31ed86e9ddcf:/go/src/github.com/docker/docker# docker version
 bash: docker: command not found
```
You will create one in the next steps.
From the /go/src/github.com/docker/docker directory make a docker binary with the make.sh script.
```
 root@5f8630b873fe:/go/src/github.com/docker/docker# hack/make.sh binary
```
You only call hack/make.sh to build a binary inside a Docker development container as you are now. On your host, you'll use make commands (more about this later).
As it makes the binary, the make.sh script reports the build's progress. When the command completes successfully, you should see the following output:
---> Making bundle: binary (in bundles/1.5.0-dev/binary) Created binary: /go/src/github.com/docker/docker/bundles/1.5.0-dev/binary/docker-1.5.0-dev

List all the contents of the binary directory.

 root@5f8630b873fe:/go/src/github.com/docker/docker#  ls bundles/1.5.0-dev/binary/
 docker  docker-1.5.0-dev  docker-1.5.0-dev.md5  docker-1.5.0-dev.sha256

You should see that binary directory, just as it sounds, contains the made binaries.

Copy the docker binary to the /usr/bin of your container.

 root@5f8630b873fe:/go/src/github.com/docker/docker#  cp bundles/1.5.0-dev/binary/docker /usr/bin

Inside your container, check your Docker version.
```
 root@5f8630b873fe:/go/src/github.com/docker/docker# docker --version
 Docker version 1.5.0-dev, build 6e728fb
```
Inside the container you are running a development version. This is the version on the current branch. It reflects the value of the VERSION file at the root of your docker-fork repository.
Start a docker daemon running inside your container.
```
 root@5f8630b873fe:/go/src/github.com/docker/docker#  docker daemon -D
```
The -dD flag starts the daemon in debug mode. You'll find this useful when debugging your code.
Bring up one of the terminals on your local host.
List your containers and look for the container running the dry-run-test image.
```
$ docker ps
```
In this example, the container's name is tender_shockley; yours will be different.
From the terminal, start another shell on your Docker development container.
```
$ docker exec -it tender_shockley bash
```
At this point, you have two terminals both with a shell open into your development container. One terminal is running a debug session. The other terminal is displaying a bash prompt.
At the prompt, test the Docker client by running the hello-world container.
```
root@9337c96e017a:/go/src/github.com/docker/docker#  docker run hello-world
```
You should see the image load and return. Meanwhile, you can see the calls made via the debug session in your other terminal.

Restart a container with your source

At this point, you have experienced the “Docker inception” technique. That is, you have:

built a Docker image from the Docker repository
created and started a Docker development container from that image
built a Docker binary inside of your Docker development container
launched a docker daemon using your newly compiled binary
called the docker client to run a hello-world container inside your development container

When you really get to developing code though, you'll want to iterate code changes and builds inside the container. For that you need to mount your local Docker repository source into your Docker container. Try that now.

If you haven't already, exit out of BASH shells in your running Docker container.
If you have followed this guide exactly, exiting out your BASH shells stops the running container. You can use the docker ps command to verify the development container is stopped. All of your terminals should be at the local host prompt.
Choose a terminal and make sure you are in your docker-fork repository.
```
 $ pwd
 /Users/mary/go/src/github.com/moxiegirl/docker-fork
```
Your location will be different because it reflects your environment.
Create a container using dry-run-test, but this time, mount your repository onto the /go directory inside the container.
```
 $  docker run --privileged --rm -ti -v `pwd`:/go/src/github.com/docker/docker dry-run-test /bin/bash
```
When you pass pwd, docker resolves it to your current directory.
From inside the container, list your binary directory.
```
 root@074626fc4b43:/go/src/github.com/docker/docker# ls bundles/1.5.0-dev/binary
 ls: cannot access binary: No such file or directory
```
Your dry-run-test image does not retain any of the changes you made inside the container. This is the expected behavior for a container.
In a fresh terminal on your local host, change to the docker-fork root.
```
 $ cd ~/repos/docker-fork/
```
Create a fresh binary, but this time, use the make command.
```
 $ make BINDDIR=. binary
```
The BINDDIR flag is only necessary on Mac OS X but it won't hurt to pass it on Linux command line. The make command, like the make.sh script inside the container, reports its progress. When the make succeeds, it returns the location of the new binary.
Back in the terminal running the container, list your binary directory.
```
 root@074626fc4b43:/go/src/github.com/docker/docker# ls bundles/1.5.0-dev/binary
 docker	docker-1.5.0-dev  docker-1.5.0-dev.md5	docker-1.5.0-dev.sha256 
```
The compiled binaries created from your repository on your local host are now available inside your running Docker development container.
Repeat the steps you ran in the previous procedure.
- copy the binary inside the development container using cp bundles/1.5.0-dev/binary/docker /usr/bin
- start docker daemon -D to launch the Docker daemon inside the container
- run docker ps on local host to get the development container's name
- connect to your running container docker exec -it container_name bash
- use the docker run hello-world command to create and run a container inside your development container

Where to go next

Congratulations, you have successfully achieved Docker inception. At this point, you've set up your development environment and verified almost all the essential processes you need to contribute. Of course, before you start contributing, you'll need to learn one more piece of the development environment, the test framework.