Automate cypress runs for PRs

[thekaveman]

#122 introduced an initial Integration/End-to-End test setup, but the only way to run tests right now is on localhost by hand.

We'd like to instead run the tests on each PR as a merge check in GitHub Actions. Similar to the work over in cal-itp/eligibility-server#16.

Basic outline is:

• on PR to any branch
• Start benefits container, see https://docs.github.com/en/actions/creating-actions/creating-a-docker-container-action
• Configure CYPRESS_baseUrl and other needed environment
• Run cypress tests, see https://github.com/cypress-io/github-action