Fix EXPOSE cache miss issue

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume@charmes.net> (github: creack)
Add unit test for expose cache

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume@charmes.net> (github: creack)
Fix sphinx header underline warnings I introduced...

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Make sphinx warnings fatal in Travis

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4664 from tianon/fix-sphinx-warnings

Fix sphinx warnings
Merge pull request #4653 from creack/fix_expose_cache

Fix expose cache