Combine the dockerfiles

[dannystaple]

This requires #205 and #206 to be completed.
If the dockerfile for serving is used to produce site files, and staging test as a further stage to load the files, then these should be combined to a single dockerfile in the root, with targets for serving, and for the staging test.

Keeping the additional staging test config in the scripts folder, combine the dockerfiles into a single multistage docker file, with:

• a base target for getting NPM going (this can be used for the compose operations like serve)
• a build target for doing the dist and site build. (intended for workflows, since it makes the site build) - can we use docker copy to copy out the site folder and put it into the tarball here?
• A staging test target for the whole http business, althuogh it should use a tagged version of the http container.
  • This can use docker copy from instructions to copy the _site folder over as needed, plus the http conf.

This will mean that ci workflows and compose file change, but should change to become a simpler and more unified setup.
README files may also need updating.