Update python_web_app.rst

Fixed typo in the WEB_PORT command, missing sudo in front of docker command
Unable to find image error should print to stderr
added a Dockerfile which installs all deps and builds the docs.
Merge pull request #1739 from dotcloud/add-docs-dockerfile

Added a Dockerfile which installs all deps and builds the Docs
hide version when not available
Updated "Use -> The Basics" to use ubuntu:12.10.

ubuntu:latest doesn't have nc. ubuntu:12.10 does.
Merge pull request #1736 from ramonvanalteren/patch-1

Update python_web_app.rst
Install Ubuntu raring backported kernel from official archives directly.
Document FROM <image>:<tag> Dockerfile instruction.
Merge pull request #1743 from dotcloud/fix_version

Hide version when not available
Merge pull request #1738 from jonasi/error-to-stderr

Unable to find image error should print to stderr
Merge pull request #1749 from msiebuhr/doc-from-tags

Document FROM <image>:<tag> Dockerfile instruction.
Merge pull request #1744 from dsissitka/patch-7

Updated "Use -> The Basics" to use ubuntu:12.10.