I’m looking for any tips/tricks or advice as I take this on. I’ll update this thread if/when I find something that works, as well (so I can find it when I google for it later!)
running Protractor tests against test/beta/prod environments (so not spinning up the app to test it)
running the tests on a docker image (not a grid or service)
This actually sounds like a stackoverflow question Hilary, I know stackoverflow is like the grammar-police, but you are asking a dev-ops question, and the answers there are far too often lacking real context. But try anyway.
Some things you already know:
Circle CI, is mostly just Jenkins but with a support contract
Docker is a sandbox for processes, and easy to use. install it locally and create experiments before moving forward again
Protractor, well I assume you want to run your tests in a browser of some sort, might be difficult, since Docker is by nature totally headless. so not something I would try, but still worth asking for alternatives