Continuous integration workflow does not work for external contributors

[gianarb]

The workflow requires access to Datum Container Registry, it means the CI check will fail for external contributor working with their own fork.

I am not sure how we want to fix this, but it sounds unusual for a CI check that runs in a PR to push to a container registry. I understand it is probably to be able to get an easy preview of the website via docker image.

/cc @scotwells not sure who should work at it

[ronggur]

Thanks @gianarb for flagging this, I've already patched the workflow. It should be much smoother for external contributors now.