ci: move the container build process to bake

[NiccoloFei]

Switching the build process to bake, creating 3 targets by default: minimal, standard and postgis.
This is an initial effort needed in order to allow running PG Major Upgrade E2E tests also in this repository, which will allow us to test major upgrades to development versions of PG18. The test's are failing at the moment for many reasons, mainly because the target images are not available. See any of the latest CD runs.

In CNPG:

• modify the E2Es to support customizing the targetImage's repositories that are part of the buildScenario, allowing us to point to postgresql-trunk

Note:

1. barman-cloud should eventually be removed from the standard target, which adds barman-cloud and pgaudit(currently commented out), once plugin-barman-cloud becomes stable
2. The postgis target can also eventually be removed once extensions can be loaded via ImageVolume, but at the moment it's needed