Rails 5 Upgrade - Update Staging to run on Rails 5 (building from Dockerfile.rails-next

[yuenmichelle1]

• Set canary replicas to 0.
• Build staging off of dockerfile.rails-next

Here is a similar PR on panoptes to give a bit more context on process (and can be used as a comparison for review) https://github.com/zooniverse/panoptes/pull/4111/files