swh-apps-build-docker-image: Make basic job build and push image

For now, this only allows building one app image from jenkins.

Ongoing work is still being developed to trigger rebuild of images being impacted by an upstream release. It will be another job in charge of triggering in // the job here.

fwiw, i've built some images already with the local setup using exactly this. (only the registry part is not working for some local issue unrelated to the mr)

Note that i left debugging instructions to ease introspecting issues later on if any.

Ref. swh/infra/sysadm-environment#4724 (closed)