[Rpm-maint] [rpm-software-management/rpm] RFE: Standardize on OCI images for test-suite, even locally (Issue #2643)

[Michal Domonkos]

To summarize again, these are our options to resolve the ticket:

### 1) No change, keep the native & podman backends

Image building:
* Done by a host specific script, typically with a native package manager (DNF/Zypper/...)

Container layering:
* Host and per-test isolation is done with Bubblewrap & OverlayFS

Pros:
* Iterative `make shell` development, without needing to throw the container away on each rebuild
* Ability to reuse the same DNF/Zypper/... wrapper for managing software in the test tree in `make env`
* Pristine image with no pre-existing RPM artifacts or unwanted software
* Separate backends optimized for different use cases (native development vs. portability)

Cons:
* Possibly more maintenance due to the use of the native package manager
* More complicated to support other hosts natively (e.g. OpenSUSE)

### 2) Use an OCI base image

Image building:
* Done by Podman/Docker using a host specific Dockerfile

Container layering:
* Host and per-test isolation is done with Bubblewrap & OverlayFS

Pros:
* Standard base image format
* Easier to support other hosts

Cons:
* Less convenient `make env` (no host specific package manager wrapper, needs manual use, e.g. `dnf --installroot=$RPMTEST` works but is not optimal, needs more options for local cache reuse or to suppress warnings, all of which is part of `mktree.fedora` already)
* Less control over the base image (still tweakable through the Dockerfile but more work)
* Additional dependency on the host (Podman/Docker)

### 3) Use OCI for full image & container management

Image building:
* Done by Podman/Docker using a host specific Dockerfile

Container layering:
* Host isolation is done by Podman/Docker natively (`podman run`)
* Per-test isolation is done with Bubblewrap & OverlayFS

Pros:
* Single dependency on the host (Podman or Docker)
* Same stack locally and in CI

Cons:
* Less convenient and more complex `make env` (mounting `$RPMTEST` would require a different containerization stack than what's used in the test-suite)
* Much less useful `make shell` (user changes to the container are dropped on each RPM rebuild)
* Docker is missing a lot features, e.g. no equivalent for `podman image mount` or `podman unshare`

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