Support for Docker Volumes in Development Containers

[mtnbikerjoshua]

Per this comment, and the devcontainers documentation, development containers do not yet support Docker volumes.

I use a Docker volume for my renv cache so that packages don't have to be reinstalled when I rebuild my container. A Docker volume makes more sense to me than a normal mount in this case because the cache contains linux specific package binaries that I would never need on my local Mac. For this use case, support for Docker volumes would be a useful feature for me. I'm also open to suggestions for a better place to store my renv cache.

[juliasilge]

Thanks for that clear description of your renv cache use case!

The good news is that the "Docker volumes are not supported" limitation is narrower than you might be interpreting it. It refers to configurations where the workspace itself lives in a named volume (VS Code's "clone repository in container volume" style setups), not to volume mounts you declare yourself. Positron passes the mounts entries from your devcontainer.json straight through to docker create, including type=volume, so this should already work today:

{
  "image": "rocker/r-ver:4.5",
  "mounts": [
    "source=renv-cache,target=/renv-cache,type=volume"
  ],
  "containerEnv": {
    "RENV_PATHS_CACHE": "/renv-cache"
  }
}

The named volume persists across container rebuilds, so your Linux package binaries stay cached without ever touching your Mac's filesystem.

A few notes on this:

• Variable substitution (like ${localWorkspaceFolder}) inside mounts is not currently expanded, but a named volume mount doesn't need it.
• If you ever hit trouble with mounts, runArgs is also passed through verbatim, so "runArgs": ["--mount", "type=volume,src=renv-cache,dst=/renv-cache"] is an equivalent fallback.
• A plain bind mount to a local folder also works, though it sounds like the named volume is a better fit for you.

Can you give that named volume mount approach a try? If you run into any problems, please let us know.

[mtnbikerjoshua]

Thanks for clarifying that! I opened my dev container in Positron, and it worked just fine! Rather than creating the mount myself, I am using the renv-cache feature from Rocker"ghcr.io/rocker-org/devcontainer-features/renv-cache:latest": {}. See rocker-org/devcontainer-features for anyone interested.

Edit: Positron had no problem opening my prebuilt container. When I tried to rebuild the container in Positron, I got an error. It seems like maybe the feature installation isn't running as root like it should?

==> Installing dev container features...
==> Installing feature: renv cache (0.1.2)
mkdir: cannot create directory ‘/tmp/dev-container-features/renv-cache_0’: Permission denied