Created a dockerized dev server

[moredip]

Some folks might find it easier to test out their changes to the yaml files if they can run the site locally using Docker, rather than mucking around with bundle install etc

[moredip]

This is now mostly done (see README), but /specs and /client are b0rk3d for some reason. Need to investigate.

[moredip]

/specs works (as long as you remember to do a git submodule update --init --recursive), but /client still doesn't. I think this might be to do with npm being run as root inside the container (per this thread).