Rucio 1.31.0rc1 feature release candidate

build: allow builds of latest custom_tags together

if the build context contains 'push-latest' and 'custom-tag=something'
in the spec, then image_tag-<custom_tag> and latest_tag-<custom_tag>
will both be pushed to DockerHub.
For example,
'fts-cron/Dockerfile_java, push-tagged, push-latest, custom-tag=java'
will push rucio/fts-cron:1.31.0-java, rucio/fts-cron:latest-java