add domain socket ability for DB conn

This allows the container to use a host's DB server via its Unix-domain socket rather than its IP.  It involves sharing the socket file via a volume mount, and specifying the path within the container to that socket via the DB_SOCK environment variable.

Note: I've tested this with MySQL and it works very well.  The logic is there for PostgreSQL as well, but I have not tested it.

This pull request is also re-based to your latest commits to master.  It merges with the use-ssl pull request with only the Changelog.md lines conflicting.