When the workspace/git checkout is a container volume, it can't be
mounted with a regular bind mount, as the compose config needs to know
the volume name, mount point, and subdirectory within the mount point to
use as the working directory.

This commit allows these 3 options to be configured using environment
variables, so that a devcontainer config can override the guidebook
service mounts to reference the devcontainer's volume. The defaults
continue to use the local bind mount, so using the compose config without
additional configuration will work as before.