Add Docker disk space troubleshooting guide

Provides diagnosis and troubleshooting steps to assist
users who encounter Docker container failures following
the event where /var partition fills up.

Change-Id: I53bcb6e447568f52fb0a16865a4b7292e7cedb50
Closes-Bug: #1383741
(cherry picked from commit c69cb2ce441b74e09a11afe70a4d733837d753f4)