The last rebuild only promoted our Gerrit 3.8 image. This appears to
have happened because we only modified the jobs and not the Dockerfile
itself. Fix this by modifying the Dockerfile which should rebuild and
promote both 3.8 and 3.9 images ensuring that our upgrade testing tests
what we want to upgrade to.
Change-Id: I8d06ea9971a6ee0c0e06e6fe2b73391526be6220