CI: prefix all tags with `fdo-ci-` so we can clean up the registry

fixes #5 (closed)

Not 100% sure this is the correct way of telling the janitor we want these to be cleaned. I wonder if we should change the image name only or if we can just change the tags.

The documentation is not clear to me as they mix images and tags:

Docker tags with names matching this regex pattern will expire:

But the example gives:

Wildcards such as .*-stable or production/.* are supported. To select all tags, use .*