ci: re-build docker image if dependent files changed

[glimchb]

we already do the same with qcow

[karlatec]

Two things here:

• You're building the image, but it's not going to be used. push_to_registry is not set, so you'd just build the image and... that's it. It would not be used in future test runs.
• Wouldn't this push event start building Qcow and Docker images at the same time? build_docker.yml uses latest available Qcow2 image as base. But if both jobs start at the same time then the latest Qcow2 used is actually the old one, from before the push event, because the new one haven't had time to finish building. I think these should be run sequentially:
  • push event comes
  • start building new Qcow2 image
  • Qcow2 build successful; start building new Docker image
  • Docker image build successful; push it to registry