The ``docker-compose.yml`` is saved along with an executable ``dc`` and
``.env`` in a directory to be shared as a volume in case we'd like to
launch a subservice in the same setup from a launched service. For instance,
a ``cron`` charm could want to launch ``run-once`` services.
The charm only need to access the ``dc`` binary, this can be done by
sharing the binary as a volume. This is the recommended way to do this:
``\${DOCKER_COMPOSE_PATH}/bin/dc:/bin/dc:ro``. Notice how we use a
docker-compose level variable here as the DOCKER_COMPOSE_PATH will only
be computed after all ``init`` hooks are done. This is the purpose of
the ``.env`` file that is also generated.
These directories are garbage collected if no containers have volumes
connected to it.
Valentin Lab