build-docker-image: Introduce a testing stage
This also transforms the 'cleanup' stage into an always post block to trigger whatever the results of previous stages is.
Related to swh/infra/sysadm-environment#3746 (closed) Related to swh/devel/fixtures/maven-index-exporter!5 (closed)
Test Plan
tox
Migrated from D7511 (view on Phabricator)