As previously discussed in moby/moby#4830, it would be useful to be able to bind-mount relative paths like this:

docker run --mount type=bind,source=host-dir,target=/app ...

Note: The original issue suggests docker run -v host-dir:/app ... which might (probably?) be confused with named volumes so I think it's more likely to expect it from the newer --mount option.

The two main motivations for this are:

• Consistency with docker-compose.
• Ability to write cross-platform docker run commands, e.g., we have a npm script that currently does something like "start": "docker run -v $PWD:/app ...". It's not easy at all to write a command that would work across Linux, macOS and Windows today.