move database storage onto persistent volume outside of docker, allowing r/w access from within docker

Currently, table storage is wherever ubuntu's mysql sticks it by default, inside of the container since that's where mysql is installed. The total size of storage needed is minimal; 200MByte would probably have us set for life. I suspect we need to reconfigure the ubuntu mysql package (inside the container, probably during image build) to point into something like /opt/shift-data/mysql and expose that directory or its parent to docker in the startup command. For bonus points, add some documentation around where it is and what is needed from a local dev to prepare their dev machine's filesystem to use this.

Start with Dockerfile line 8 and the Ubuntu MySQL package configuration. Trace how the startup command exposes storage, then determine how /opt/shift-data/mysql should be used for persistent access from inside Docker. Done means MySQL data survives container replacement and the issue includes documentation for preparing the local filesystem.