Add tools for accessibility and end to end testing, restructure repo for more images, reinstate CI

[tomgp]

We want to be able to test performance, accessibility, and run automated end to end testing on sites

Installing chrome/ puppeteer etc during CI

1. Can significantly add to the build time
2. I can never get it to work 😄

So this PR adds the required tooling to the Docker image: Chrome, associated font rendering packages and then pa11y-ci which installs Puppeteer as a dependency.

Note setting PUPPETEER_SKIP_CHROMIUM_DOWNLOAD to true so that pa11y etc. won't try to install the latest version which apparently won't work with Alpine

[marcelkornblum]

I tihnk it'd be worth doing this as a new image other than base - though we need to restructure the folders to make that happen (which I'm in favour of anyway)