How to Share a Volume With a Specific Docker Container in Visual Studio Codespaces

Visual Studio Codespaces provides cloud-powered development environments for any activity - whether it’s a long-term project, or a short-term task like reviewing a pull request

I use it to have a development environment dedicated to a repo.
I like it because it provides me with a Linux environment that includes most of the tools I need,
including Docker.

You’ll find more information in the documentation at https://docs.microsoft.com/en-us/visualstudio/codespaces/overview/what-is-vsonline.

One of the things I found tricky was to docker run with a volume that points back to my development environment.

what is the problem?

To illustrate the issue, I created a sample repo: https://github.com/benjguin/sample200902

I create a Visual Studio Codespace for it.
I go to https://online.visualstudio.com/environments, then Create Codespace with the following parameters:

From the command line part of Visual Studio (CTRL-`), I have:

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codespace:~/workspace/sample200902$ docker ps -a
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS               NAMES
443c621d0ef5        1da2d08caca9        "/usr/local/share/do…"   44 seconds ago      Up 42 seconds                           codespaces_5d85d6

The container I see (443c621d0ef5) happens to be the container from which the command line executed.
In other words, the development environment is a container, not the docker host itself.

So, if I try to run docker with a volume pointing back to my current folder, I do not see the local folder content.
Let’s try:

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codespace:~/workspace/sample200902$ docker run -it --rm -v $PWD:/my-vol alpine sh
/ # ls -al /my-vol
total 8
drwxr-xr-x    2 root     root          4096 Sep  2 16:33 .
drwxr-xr-x    1 root     root          4096 Sep  2 16:33 ..

Why? Because $PWD points to current working dir on the host, not on the dev container.

How to workaround it?

Both development environment and container can meet in a common folder, on the docker host.

By inspecting our development environment we can find one:

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codespace:~/workspace/sample200902$ docker ps -a
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS               NAMES
443c621d0ef5        1da2d08caca9        "/usr/local/share/do…"   35 minutes ago      Up 35 minutes                           codespaces_5d85d6
codespace:~/workspace/sample200902$ docker inspect 44 | grep containerTmp --before=5 --after=5
                "/root/.vsonline/.vsoshared:/home/codespace/.vsonline/.vsoshared",
                "/.vsonline/vsoagent/mount:/.vsonline/bin",
                "/var/lib/docker/vsonlinemount/workspace:/home/codespace/workspace",
                "/var/run/docker.sock:/var/run/docker.sock:ro",
                "/usr/bin/docker:/usr/bin/docker:ro",
                "/mnt/containerTmp:/tmp"
            ],
            "ContainerIDFile": "",
            "LogConfig": {
                "Type": "json-file",
                "Config": {}
--
                "RW": false,
                "Propagation": "rprivate"
            },
            {
                "Type": "bind",
                "Source": "/mnt/containerTmp",
                "Destination": "/tmp",
                "Mode": "",
                "RW": true,
                "Propagation": "rprivate"
            },

basically, /mnt/containerTmp on the host is /tmp in the dev container.

So let’s try to mount /mnt/containerTmp in the docker container to see if we can see what’s in the dev container’s /tmp folder:

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codespace:~/workspace/sample200902$ cp some-content.txt /tmp/content-copy.txt
codespace:~/workspace/sample2009$ docker run -it --rm -v /mnt/containerTmp/:/my-vol alpine sh
/ # ls /my-vol/*.txt
/my-vol/content-copy.txt
/ # cat /my-vol/content-copy.txt
hey, I'm a text file in the dev environment.
/ #

It works. Here is what happened:

  • Host
    • dev container
      • /tmp/content-copy.txt -> Host /mnt/mnt/containerTmp/content-copy.txt
    • alpine container
      • /my-vol/content-copy.txt -> Host /mnt/mnt/containerTmp/content-copy.txt

I had to use that in a project where we needed to interact with binary files from within the container, and be able to test from within the development environment.
This trick helped me!

:-) b