support gitlab-runner as a "runtime"

Instead of running unfurl inside a python virtual machine we should add an option that runs it via gitlab runner.

It might be possible to do this out of the box: if you specify a runtime (either on the command line with --runtime or with a runtime key in contexts in unfurl.yaml) that isn't a venv: specifier it will treated as command line arguments that prepend the arguments passed to unfurl. (see _remoteCmd in __main__)

There's a simple Dockerfile for building a container with unfurl development environment but we probably want a better container for a gitlab-runner instance to use.