Mock as tool to prepare container image with rpm-build only

[xsuchy]

Short description of the problem

Mock nowadays prepares a buildroot environment and then calls rpmbuild inside this environment. It would be useful if we could separate these two steps and use Mock to prepare the environment and create the container image. Optionally upload this container image to a registry.
This container has resolved and installed build dependencies and installed rpm-build. And it can be consumed by other tooling or by another Mock command (described in #1159)

This image should be created in the prebuild phase. We should likely create a new phase, e.g., preimage_create.

And I think we should allow the user to stop after the image is created and not continue with the rest of the build process.

[praiskup]

This image should be created in the prebuild phase.

There's the new postdeps hook.