Break dependency on packit-worker image

[jpopelka]

For now, we have only prod deployment of the fedora source-git bot but the prod image is always built FROM packit-worker:latest.
That's good enough for proof of concept, but long term we probably don't want to build FROM packit-worker, but from packit/base.
So the task here is to break the dependency on the packit-worker image. (probably something similar as we did in packit/packit-service#717)