Cached Docker images in GitHub Action

[deliaBlue]

Currently, it takes between 20-24 minutes to pull and convert the needed Docker images.
Refactor the CI workflow to run cached Docker images and decrease the running time.

[uniqueg]

Note that this is a low priority issue. You can spend some time exploring what options there are, if any, and then we can see if there's anything that we could reasonably implement.

But one way or the other, tests will run several minutes at least, and we will need to deal with that.

Also, typically the local development environment shouldn't be much different from that on the CI virtual machine, so if you only push your commits after tests have passed locally (which should be much faster), in the vast majority of cases, the outcome of the CI tests will not differ from that of the local ones.