Add a Makefile target for building html docs using a rawhide environment

This makes it easier to generate new documentation for
http://weldr.io/lorax/

It requires having a current welder/lorax-composer:latest image (created with
the test-in-docker target), then run docs-in-docker to rerun sphinx with
the docs/html directory mounted from the container.

Makefile

@@ -70,6 +70,9 @@ local:
 test-in-docker:
 	sudo docker build -t welder/lorax:latest -f Dockerfile.test .
 
+docs-in-docker:
+	sudo docker run -it --rm -v `pwd`/docs/html/:/lorax/docs/html/ --security-opt label=disable welder/lorax-composer:latest make docs
+
 ci: check test
 
 .PHONY: docs

docs/html/README

@@ -0,0 +1,5 @@
+To build the docs for this branch run:
+make test-in-docker
+make docs-in-docker
+
+If you already have a welder/lorax-composer:latest docker image you can skip running 'test-in-docker'.