Update our test/CI environment to a modern browser which matches what
our users use and supports modern web APIs and ES language features.

 - Add a Dockerfile which defines an image that includes everything
   needed to run client tests and deployment steps. This image uses
   plain Alpine as the base rather than Debian as this makes it easier
   to update frequently (every 6 months) to get a current Chrome build.

 - Replace PhantomJS with Puppeteer for testing locally and on Travis.
   Jenkins uses Chromium packages from Alpine.