fix docker build

[dunn]

Including the DB setup during rails new causes ActiveRecord to attempt to initialize a database connection, which fails because there's no DB running during the image build: https://gist.github.com/dunn/8abda1b63399c41226cea28662765fc0

[dunn]

@jcoyne yes, it was a pretty extensive change, and broke docker builds, which is why i'm proposing we revert it. if we want to remove the hard dependencies on postgres gems, that can be a follow-up PR, tested against the docker builds.

[dunn]

to that end, added a workflow step that just checks to make sure the docker image builds

[dunn]

@jcoyne's fix works, so I've updated the PR. Though for the record, I would have appreciated reverting the breaking commit and then providing a working replacement.