Enable building docs in the Docker image (includes Sphinx version bump to 1.7.1) (#4729)

* Added docs requirements to Docker image

* sphinx_celery requires Sphinx 1.7.1

pip-9.0.1 didn't complain about an older version being installed, but
pip-10.0.1 does.

docker/Dockerfile

@@ -58,6 +58,7 @@ RUN pyenv local python2.7 python3.4 python3.5 python3.6
 # Setup one celery environment for basic development use
 RUN pyenv exec pip install \
   -r requirements/default.txt \
+  -r requirements/docs.txt \
   -r requirements/pkgutils.txt \
   -r requirements/test.txt \
   -r requirements/test-ci-base.txt \

requirements/docs.txt

@@ -1,4 +1,4 @@
 git+https://github.com/celery/sphinx_celery.git
-Sphinx==1.6.5
+Sphinx==1.7.1
 typing
 -r extras/sqlalchemy.txt