readme: move link to official postgresql image to the postgresql database section
readme: move link to official postgresql image to the postgresql database section

sameersbn/docker-gitlab-ci,outcoldman/docker-gitlab-ci,bboehmke/docker-gitlab-ci,bboehmke/docker-gitlab-ci,adiferd/docker-gitlab-ci,youhong316/docker-gitlab-ci,youhong316/docker-gitlab-ci,adiferd/docker-gitlab-ci,outcoldman/docker-gitlab-ci,sameersbn/docker-gitlab-ci