Create "pod.db" under /db, mount /db to container

See merge request memri/pod!32

reduce docker layer size

See merge request memri/pod!31

docker cache dependencies

See merge request memri/pod!30

Make Pod compatible with old sqlcipher versions

See merge request memri/pod!29

In particular, make it compatible with version 3.4, as used in ubuntu-20.04.

This development instruction is part of the README now

See memri/pod#94

See memri/pod#94