Add Docker instructions for older branches

If you've upgraded away from Ubuntu 14.04 Trusty, you may find that
older branches (or certain code) doesn't build anymore. These
instructions can form a starting point to build these older branches
within Docker.

Bug: 72993575
Test: Follow instructions, build marshmallow-dev
Test: Follow instructions, build lollipop-mr1-dev
Change-Id: If4047b1bb5324d75a9d0947cb5280ff1cabccb6a