Fix odd issues with "make docs", add "make docs-shell", and canonicalize...

... our docs Dockerfile a bit more

/cc @vieux

For reference, here's the error @vieux and I were running into when running `make docs` twice (from SimpleHTTPServer itself of all things - not even Sphinx):
```
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/usr/lib/python2.7/SimpleHTTPServer.py", line 220, in <module>
    test()
  File "/usr/lib/python2.7/SimpleHTTPServer.py", line 216, in test
    BaseHTTPServer.test(HandlerClass, ServerClass)
  File "/usr/lib/python2.7/BaseHTTPServer.py", line 589, in test
    port = int(sys.argv[1])
ValueError: invalid literal for int() with base 10: '/bin/sh'
```