.env variable in the mounts section of devcontainer.json

Question

setting up a .devcontainer configuration ...

I have a mount that normally lives in one spot, but when working remotely, can live in a different path. I want to set up a devcontainer so that I can change an environment variable locally, without making a commit change to the repos. I can't change the target because it is a convention that spans many tools and systems.

"mounts": [
  "type=bind,source=${localEnv:MY_DIR},target=/var/local/my_dir,readonly"
]

For example, in a .env file in the project root:

MY_DIR=~/workspace/my_local_dir

When I try this, no matter what combinations, this environment variable always ends up blank, and then Docker is sad when it gets a --mount configuration that is missing a source:

[2022-02-12T19:11:17.633Z] Start: Run: docker run --sig-proxy=false -a STDOUT -a STDERR --mount type=bind,source=,target=/var/local/my_dir,readonly --entrypoint /bin/sh vsc-sdmx-9c04deed4ad9f1e53addf97c69727933 -c echo Container started
[2022-02-12T19:11:17.808Z] docker: Error response from daemon: invalid mount config for type "bind": field Source must not be empty.
See 'docker run --help'.

The reference shows that this type of syntax is allowed for a mount, and that a .env will be picked up by VSCode. But I guess it only does that for the running container and not VSC itself.

How do I set up a developer-specific configuration change?

Answer 1

The error message

invalid mount config for type "bind": field Source must not be empty

indicates that your source config appears empty to docker, which means that ${localEnv:MY_DIR} resolves to an empty string, which in turn means that MY_DIR isn't set on your host, because "Unset variables are left blank.".

If you set MY_DIR on your host then its path should be mounted into the devcontainer with the mount command you've used:

"mounts": [
  "type=bind,source=${localEnv:MY_DIR},target=/var/local/my_dir,readonly"
]

Defining a variable in an .env file in the project root, however, means that this variable gets set as an environment variable in the devcontainer, not locally.