fetch commit_sha from source_code_directory (don't assume we're in the source tree anymore),
allow custom tag (if a parameter is passed in as $1)
update README.md
This makes the default public port for starting FDB processes the same as the FDB_PORT. This is probably necessary given #1714, especially for coordinators, though it might not be necessary for other processes in the cluster. This can *almost* be used to start up multiple FDB processes locally and then access them from the same machine, but that (unfortunately) requires both the other processes in the docker compose network and the host machine to agree on what IP to use for the coordinator. But as that machine has different IPs in those networks, they cannot be made to agree.
Aaron Molitor