Cache backend and frontend builds in GitHub actions so we don't have to rebuild them in the build_image stage

[banool]

We build both the frontend and the backend in order to run the tests, but then we rebuild them to push the final image. We could avoid having to do this if we cached their output.

The build_image job currently uses docker based caching, but the tests can't benefit from that. Perhaps we could run the tests in a container instead and then benefit from the same cache.