# 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 Docker Machine 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, 1. Verify that you have no unnecessary containers running on your host. $ docker ps You should see something similar to the following:

CONTAINER ID	IMAGE	COMMAND	CREATED	STATUS	PORTS	NAMES

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. 2. Verify that your host has no dangling images. $ docker images You should see something similar to the following:

REPOSITORY	TAG	IMAGE ID	CREATED	VIRTUAL SIZE

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 $(docker images -q -a -f dangling=true) This command uses `docker images` to list all images (`-a` flag) by numeric IDs (`-q` flag) and filter them to find dangling images (`-f dangling=true`). Then, the `docker rmi` command forcibly (`-f` flag) removes the resulting list. To remove just one image, use the `docker rmi ID` command. ## Build an image If you followed the last procedure, your host is clean of unnecessary images and containers. In this section, you build an image from the Docker development environment. 1. Open a terminal. Mac users, use `docker-machine status your_vm_name` to make sure your VM is running. You may need to run `eval "$(docker-machine env your_vm_name)"` to initialize your shell environment. 3. 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. 4. 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. 5. 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 6. List your Docker images again. $ docker images You should see something similar to this:

REPOSTITORY	TAG	IMAGE ID	CREATED	VIRTUAL SIZE
dry-run-test	latest	663fbee70028	About a minute ago
ubuntu	trusty	2d24f826cb16	2 days ago	188.3 MB
ubuntu	trusty-20150218.1	2d24f826cb16	2 days ago	188.3 MB
ubuntu	14.04	2d24f826cb16	2 days ago	188.3 MB
ubuntu	14.04.2	2d24f826cb16	2 days ago	188.3 MB
ubuntu	latest	2d24f826cb16	2 days ago	188.3 MB

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 Docker Hub 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. 1. 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 "$(docker-machine env your_vm_name)"` in any new terminals. 2. 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.  3. 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. 4. 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 5. 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. 6. 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 7. 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. 8. 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. 9. Bring up one of the terminals on your local host. 10. List your containers and look for the container running the `dry-run-test` image. $ docker ps

CONTAINER ID	IMAGE	COMMAND	CREATED	STATUS	PORTS	NAMES
474f07652525	dry-run-test:latest	"hack/dind /bin/bash	14 minutes ago	Up 14 minutes		tender_shockley

In this example, the container's name is `tender_shockley`; yours will be different. 11. 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. 12. 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. 1. 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. 2. 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. 3. 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. 4. 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. 5. In a fresh terminal on your local host, change to the `docker-fork` root. $ cd ~/repos/docker-fork/ 6. 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. 7. 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. 8. 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](/project/test-and-docs/).