WIP: Add 2 helpers for adding extension more easily

[mscherer]

This PR should go along #394, that as I as a basis to get postgis in postgresql.

I used that dockerfile:

FROM registry.fedoraproject.org/f33/postgresql
USER 0
RUN mkdir -p postgresql-cfg && echo "shared_preload_libraries='postgis-3'" > postgresql-cfg/postgis.conf
RUN mkdir -p postgresql-init && echo -en 'echo "CREATE EXTENSION IF NOT EXISTS postgis;" | psql' > postgresql-init/enable_postgis.sh
RUN dnf install --setopt=install_weak_deps=False --best -y postgis && dnf clean all
USER 26

However, with the addition of the 2 helpers, I think this could be simplified to be:

FROM postgres-fedora
USER 0
RUN dnf install --setopt=install_weak_deps=False --best -y postgis && dnf clean all
RUN load_pg_library postgis-3 
RUN create_extension postgis 
USER 26

This permit to hide implementation details, and avoid cut and paste (since postgis provides 4 extensions), and stupid errors (i took me a while to get a working container).

I marked the PR as WIP since there is no doc as I do not know where you would like to add it, and there is also no test (but I guess this can be done with pgaudit).

[centos-ci]

Can one of the admins verify this patch?

[centos-ci]

Can one of the admins verify this patch?

[centos-ci]

Can one of the admins verify this patch?

[mscherer]

So, upon further experimentaion, I think the part about creating the extension may not always work (as I need to create it per database, and I do not know enough of postgres if that's a good or bad idea). I guess we could add a 3rd hook for that (create in the database).

Is it safe to assume that the container will be used with 1 single database, and not shared accross multiple pod with multiple db ?

[mscherer]

Replaced by https://github.com/sclorg/postgresql-container/pull/399