Add containerized development environment

[tedzards509]

Apart from the GitHub action, this is all tested.

I created a new Dockerfile that runs yarn dev and compose file that mounts the relevant directories into a development environment.

The advantages of this are that contributors don't have to juggle node versions or packages and the onboarding process is (IMO) friendlier because you don't have to run code on bare metal, nor install systemwide dependencies.

Then I modified the GitHub action that builds the deployable image to also build a dev image. I couldn't test the changes but followed this comment quite closely, so I hope it just works.