Docker build should run psql tests, requires psql service in github actions

[mgrunbauer]

Adaguc supports both PostgreSQL and SQLite as database back-ends, however we want to deprecate SQLite and only support PostgreSQL.

The PR #361 adds the file runtests_psql.sh which can already run the tests against PostgreSQL during the docker build. This works when building adaguc on a local system. Note that this requires a running PostgreSQL server reachable on port 5432.

We should try using a PostgreSQL service in GitHub Actions. This would allow us to run all tests against PostgreSQL. If this works, we can remove support for SQLite.