Fix acceptance image test

[davisagli]

Reported by @fredvd in #164 (comment):

we need to fix that acceptance image test, as it is pushing the base images in the test to hub.docker.com, factually publishing them before the real build and deploy to hub.docker.com is happening later. :-P

(for example build and push the base ones to test to GHCR instead)

Also: the test acceptance for plone-backend passed where subsequently the build and deploy to hub.docker.com Action failed last week.....