Allow mounting subdirectories of $BITBUCKET_CLONE_DIR for Docker run.

Currently when using the Docker run command, it is only possible to mount the $BITBUCKET_CLONE_DIRECTORY. It would be good to allow support for mounting other directories under the $BITBUCKET_CLONE_DIRECTORY

Allow for mounting directories such as:

#!
$ docker run -v $BITBUCKET_CLONE_DIR/app:/app my-container:version

As per this discussion here. Including a few example use-cases: https://community.atlassian.com/t5/Bitbucket-questions/Re-Re-Bitbucket-pipelines-how-can-I-map-a-volume-to-do/qaq-p/703437/comment-id/22253#M22253