readme: remove links to quay registry
readme: remove links to quay registry

The `assets:compile` rake task requests more memory than the quay.io
build machine can provide. As a result builds on quay.io have been
failing for a long time now.

sameersbn/docker-gitlab,solidnerd/docker-gitlab,solidnerd/docker-gitlab,sameersbn/docker-gitlab,3kami3/docker-gitlab,3kami3/docker-gitlab