Support `--socket-dir` command line argument

[tjhorner]

The dispatcher server accepts a --socket-dir command line argument to let the user customize the location of the unix domain sockets that are created. The dispatcher client already listens to OVERPASS_SOCKET_DIR, so it would be great if dispatcher_start.sh added the appropriate --socket-dir parameter if OVERPASS_SOCKET_DIR is set. I think some additional work would need to be done for the dispatcher_areas supervisord job and here in the entrypoint.

By default, Overpass stores the domain sockets in the database directory, but this can cause problems if the directory is a network share or otherwise does not support unix domain sockets, so specifying a separate directory for those is required.

Thank you for this Docker image!