Add KB instructions on SE testing environment
Fixes https://gitlab.com/gitlab-com/support/issues/54 - replacing https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/3098
Requires the completion of https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/3381
- Instructions on using VMware + Docker
- Instructions on using Digital Ocean + Docker
- Include installation of Docker + Docker-Machine
- Installation of GitLab custom version
cc// @gl-support