Image build pipeline

Create an image build pipeline that only includes the module files (to be used as an extension with an image volume). If this is not possible, it is acceptable in this phase to add it as a layer on top of the minimal Postgres image.

Images must be built for the supported Debian distributions (trixie and bookworm) and Postgres versions. Use this tag: `postgres-keycloak-oauth-validator:18.0-0.0.1-YYYYMMDDHHmm-trixie`, where 0.0.1 is the version of the validator.

For now, let's build the image from sources.