postgres-350d-15.4 : upgrade postgreSQL and fix locale problem in base image

PostgreSQL is now provided in 15.4 version.

There was a new problem when building the previous image, with the Debian base system not having locale properly generated. I think it may be a temporary upstream bug, but it's now handled by regenerating locales.

postgres-350d-15.3 : current postgresql and tests

postgres-350d has been updated to support current version of postgresql, the 15.3 one.

Additionally, there is now a test suite to make it easier to do incremental upgrade in the feature.

And finally, the image is now hosted on Gitlab's registry.