Shell container

Closes: https://gitlab.com/charts/helm.gitlab.io/issues/56

The shell container in this MR does function as the proper ssh server, for authorizing ssh requests and passing off to gitaly. But this MR puts unicorn into a full gitaly enabled mode, that that breaks much of the repository UI when unicorn doesn't have access to the storage.

With this in place you are able to add your ssh key to gitlab, and clone repos over ssh. Once you start pushing changes, the system will start breaking, as sidekiq and unicorn are still expecting the repo storage to be present.