Build docs in directory where they are mounted

It seems the docs are mounted in the Django development server at
build/sphinx/html, not from doc/_build . Not entirely sure when this
change took place, but the docbuild container should put the rebuilt
docs in the location where they are actually served from.

tools/docker/doc-watch.sh

@@ -5,9 +5,9 @@ cd /source
 # Build once first
 sudo -u nav python3 -m build  # ensure build data and .eggs aren't stored as root
 pip install -e .
-sudo -u nav sphinx-build doc/ doc/_build
+sudo -u nav sphinx-build doc/ build/sphinx/html/
 # Then re-build on any changes to the doc directory
 while inotifywait -e modify -e move -e create -e delete -r --exclude \# /source/doc /source/NOTES.rst
 do
-  sudo -u nav sphinx-build doc/ doc/_build
+  sudo -u nav sphinx-build doc/ build/sphinx/html/
 done