Improve Docker build - Easy way to limit OS and/or Postgres versions

[rustprooflabs]

Details

RE #43 & #44.

It should be easier to customize the Docker build system to limit OS and/or Postgres selection. Current solution of hacking build.sh isn't ideal. Users should be able to define one or more OS options, as well as one or more Postgres version.

Considering adding env vars, e.g. PGDD_BUILD_OS and PGDD_BUILD_POSTGRES_VERSIONS