Currently the Gitaly images install the PostgreSQL version available
to the Debian repository. The support scripts in Praefect currently
depend on specific versions of `psql`, so let's be explicit about the
versions used in the image and install PostgreSQL from the PGDG
repository.
This refactors our custom image building to utilize GitLab parallel jobs
with a matrix.
This makes it easier to parse what kind of matrix we are going to build
in our docker images. Furthermore instead of splitting the image name,
we can simply pull the versions of the tools from the environment
variables.
The ultimative reason: Dogfooding
