Gitaly is using PgBouncer in its CI pipelines to verify that Praefect
works alright with this proxy for Postgres. While it was initially using
the PgBouncer binary directly, the project has since migrate to set up
PgBouncer as a service via a separate image in 0e5953177 (ci: Tie in
PgBouncer as a service, 2021-12-03). Since then the PgBouncer executable
is not needed anymore in the image used by Gitaly.
Stop installing PgBouncer and remove the architecture we have to build
and install it.
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
