add domain socket ability for DB connections

This patch allows the container to access a DB running on the host through a volume mount of the host DB's Unix-domain socket.  This method is slightly faster than IP (and is the preferred method when specifying an IP of localhost anyway), and allows the host configuration to keep its listening interface on localhost rather than listening on an external IP or all interfaces.

Some caveats:  This patch has not been tested since the many recent changes to the base image.  This patch was developed before them, and was recently rebased to the new changes.  While the merge was successful (with updates to the README.md and Changelog.md files), the implications of the recent base image changes have not been fully analyzed.

This had been tested before the recent changes using only a MySQL instance on the container's host.  While the ability to use PostgreSQL on the host exists, and this patch should support it, the author has not had the opportunity to test it with PostgreSQL.

Please also review the updates to the README.md file to make sure they are in the right place and reflect any updates to human procedures introduced in the recent changes.