Revert "Remove intro and link to Dockerhub"

This reverts commit 57ec2832.

docker/README.md

 # GitLab Docker images
-## What is GitLab?
-
-GitLab offers git repository management, code reviews, issue tracking, activity feeds, wikis. It has LDAP/AD integration, handles 25,000 users on a single server but can also run on a highly available active/active cluster.
-Learn more on [https://about.gitlab.com](https://about.gitlab.com)
+The GitLab docker image is [available on Docker Hub](https://registry.hub.docker.com/u/gitlab/gitlab-ce/).
 ## After starting a container