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Installing Docker in the Windows subsystem for Linux

Now that you have the Windows subsystem for Linux installed, you need to install the Docker client, Docker Compose, and other supporting tools in your distribution. In this section, I will assume that you are using the Ubuntu Xenial (16.04) distribution.

To install Docker, follow the instructions at https://docs.docker.com/install/linux/docker-ce/ubuntu/#install-docker-ce to install Docker:

> sudo apt-get update
Get:1 http://security.ubuntu.com/ubuntu xenial-security InRelease [107 kB]
Hit:2 http://archive.ubuntu.com/ubuntu xenial InRelease
Get:3 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [109 kB]
...
...
> sudo apt-get install \
    apt-transport-https \
    ca-certificates \
    curl \
    software-properties-common
...
...
> curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
OK
> sudo add-apt-repository \
   "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
   $(lsb_release -cs) stable"
> sudo apt-get update
...
...
> sudo apt-get install docker-ce
...
...
> docker --version
Docker version 18.06.0-ce, build 0ffa825
> docker info
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

In the preceding example, you must follow the various instructions to add the Docker CE repository to Ubuntu. After installation is completed, you must execute the docker --version command to check the installed version, and then the docker info command to connect to the Docker Engine. Notice that this fails, as the Windows subsystem for Linux is a user space component that does not include the necessary kernel components required to run a Docker Engine.

The Windows subsystem for Linux is not a virtual machine technology and instead relies on kernel emulation features provided by the Windows kernel that makes the underlying Windows kernel appear like a Linux kernel. This kernel emulation mode of operation does not support the various system calls that support containers, and hence cannot run the Docker Engine.

To enable the Windows subsystem for Linux to connect to the Docker Engine that was installed by Docker for Windows, you need to set the DOCKER_HOST environment variable to localhost:2375, which will configure the Docker client to connect to TCP port 2375 rather than attempt to connect to the default /var/run/docker.sock socket file:

> export DOCKER_HOST=localhost:2375
> docker info
Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 0
Server Version: 18.06.0-ce
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
...
...
> echo "export DOCKER_HOST=localhost:2375" >> ~/.bash_profile

Because you enabled the Expose daemon on tcp://localhost:2375 without TLS option earlier when you installed Docker and Windows to expose local ports to the Windows subsystem for Linux, the Docker client can now communicate with the Docker Engine running in a separate Hyper-V virtual machine that was installed by Docker for Windows. You also add the export DOCKER_HOST command to the .bash_profile file in the home directory of your user, which is executed every time you spawn a new shell. This ensures that your Docker client will always attempt to connect to the correct Docker Engine.