Mounting a volume to a target containing a colon (:)

Description

Binding a volume into a target path containing a colon (:) appears to be a missing feature. If it is possible then it's not documented. While colons are less common they have always been valid in a *nix path and are a plain old US ASKII character present on (almost?) every keyboard around the world.

There's some discussion in #12553 discussing this as a bug. But what I'm actually stuck with is the apparently missing feature. The workaround discussed in the bug ticket only applies to bind mounts by setting create_host_path. But what I'm looking for is a way to do the same for type: volume.

This is something that is possible via docker itself:

docker volume crete foo
docker run -it --rm --mount type=volume,src=foo,dst=/foo/bar:baz ubuntu

This ought to then be possible via docker compose.

As one hard example: I ran into this trying to emulate some google cloud behaviour for local testing. Google Cloud SQL will bind mount a postgres socket file into the container at something like: /cloudsql/bar-baz:europe-west4:foo-db-1234567890.

This simplest way to achieve this should be to mount a single volume into two containers:

• /var/run/postgresl in the postgres container
• /cloudsql/bar-baz:europe-west4:foo-db-1234567890 in the app container

The existence of colons in the path here is non-negotiable. I don't get to chose how google Cloud SQL works, and I do need to test that app code correctly handles connecting to this postgres socket despite the existence of colons in the DSN/URL

No repository file or test is named in the issue. Start by tracing how Compose parses and handles type: volume mounts, using Docker's --mount example as the expected behavior. Done means a Compose configuration can mount the same volume at a target containing colons in both containers.