Allow overriding host mount #601

I have a somewhat similar case to #295 and #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.