Run cypress tests against built website

[ceddlyburge]

The cypress tests on the build server, and indeed locallyl, currently run against the webpack development server.

This means that errors could creep in from differences between the webpack dev server / build and the full build.

So it would be better to npm run build to create the same code that will get deployed, use serve (or similar) to serve it and then test against that, which would remove this possibility.

[ceddlyburge]

This happens in the docker-compose.yml / usage_scenario.yml file now, but these aren't currently run as part of the build, so maybe they should be.