So st2packs-dockerfiles accepts local files/packs using the following format file:///tmp/packs/cassandra .. but apparently it still expects them to be a git repo as it fails:
2019-09-27 11:53:14,629 INFO [-] Installing pack "file:///tmp/packs/cassandra"
# cut for brevity...
git.exc.GitCommandError: Cmd('git') failed due to: exit code(128)
cmdline: git clone -v file:///tmp/packs/cassandra /root/0bd83c56596f6797b1a4c509296a3e7e
stderr: 'Cloning into '/root/0bd83c56596f6797b1a4c509296a3e7e'...
fatal: '/tmp/packs/cassandra' does not appear to be a git repository
fatal: Could not read from remote repository.
So st2packs-dockerfiles accepts local files/packs using the following format file:///tmp/packs/cassandra .. but apparently it still expects them to be a git repo as it fails: