Build Dockerfy image in CI

This MR extends !1 (merged) and builds the dockerfy image as registry.hzdr.de/kit-scc-sdm/onlinestorage/httpd-webdav/production-config/httpd-webdav-dockerfy.

There is one big open question: How can the image be tested meaningfully?

The image is not available yet in the registry because it is not yet in main. Unfortunately that also means there cannot be tests executed on it until docker-basics#9 (closed) is resolved.

/cc @Sven.Siebler