Document updating CE container to EE container

What does this MR do?

Resolves omnibus-gitlab#2994 (closed). cc @dralexx & @joshlambert

What I've done to test this:

1. docker-compose up with gitlab/gitlab-ce:latest
2. Register new user.
3. docker-compose down && sed -i '' 's/-ce/-ee/g' docker-compose.yml && docker-compose up
4. Logging in with the previously registered user: ✅

Those (DB-level) steps did not cover repository data and/or secrets, which …

• … is an open question about whether secrets.json needs to be treated especially. Logically, no, because it would be in $GITLAB_HOME/config as well 🤔

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