Allow overriding host mount

I have a somewhat similar case to https://github.com/devcontainers/cli/issues/295 and https://github.com/devcontainers/cli/issues/497 where I have a "control" container managing child instances. The control container itself is *also* in Docker, so the workspace paths there don't correspond to host paths.

Things are mostly working, except I don't want to encode the host path details in the devcontainer.json (since they differ by env). A CLI option for the host's path would be a great addition.

Would it be possible / acceptable to add a CLI flag to override the host mount path configuration?

Note: I specifically want to avoid modifying the `workspaceMount` configuration in the underlying `devcontainer.json` because the host paths change dynamically with each restart.