Drop support for quay.io. (#7187)
Drop support for quay.io. (#7187)

In this spirt of cleaning up some low hanging cruft, this fixes #4343.

There are no (recent) release tags on quay.io and the builds are just following master. See https://quay.io/repository/letsencrypt/letsencrypt?tab=tags.

Once this lands, I can disable the automated builds on quay.io and we can delete Dockerfile-old and tools/docker-warning.sh.
stweil/letsencrypt,lmcro/letsencrypt,stweil/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt,letsencrypt/letsencrypt