Add Auto DevOps compatible Dockerfile

Dylan Griffith (ex GitLab)requested to merge
dockerfile into master

Also nginx.conf to ensure it runs on port 5000 necessary for Auto DevOps. This should be compatible with Auto DevOps when we transition to deploying via Auto DevOps.

This could also be useful for running it locally if you do not want to install all the dependencies. In practice, though, this Dockerfile is mainly suited for deployments as it is not mounting the directories to enable ease of local development.

This could be combined with the MR to enable Auto DevOps but it seemed easier to keep it separate since the other one will need rebasing https://gitlab.com/gitlab-org/design.gitlab.com/merge_requests/1216

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