macOS creates metadata files named `.DS_Store`. The script incorrect tries to read it with a `.md` extension. By excluding all files that start with a dot we can prevent this.
Use the new Docker-based, beta version of CircleCI.
This adds a docker image that has the correct deps to run flow, and then migrates from the old circle.yml format to the new.
This gets test runs wayyyyy down. Earlier today, on the old stack, before optimizations, we were at > 9 min for PR builds (aka, CI runs without a website deploy). The build for this PR ran in 1 min 33 seconds. Woo!
