Add docs for using keyring in dockerized environments

[sigalsax]

The changes described in this proposal detail how to use the keyring on Linux systems (Ubuntu 18:04). I am requesting this be added to the docs because I spent a long time trying to figure this out and think others can value from this addition. The addition to the docs provides all the necessary prereqs to use keyring in a dockerized environment

[jaraco]

Thanks sigalsax for the contrib!

The test failure is likely due to this change. If you follow to a failed build, you can see these errors during the docs check:

<string>:316: (WARNING/2) Title underline too short.

Using Keyring on headless Linux systems in a Docker container
=======================================
<string>:316: (WARNING/2) Title underline too short.

Using Keyring on headless Linux systems in a Docker container
=======================================
<string>:320: (WARNING/2) Inline literal start-string without end-string.
<string>:340: (ERROR/3) Unexpected indentation.
<string>:340: (WARNING/2) Inline literal start-string without end-string.
<string>:340: (WARNING/2) Inline interpreted text or phrase reference start-string without end-string.

I'd investigate lines 316, 320, and 340 to address the warnings/errors. You can see these errors also by running tox -e docs.

Let me know if you don't feel comfortable addressing these errors and I can help.

Good luck.

[sigalsax]

@jaraco repushed and looks like everything passed now (: