Bug 1505579 - Upgrade Docker images to Mercurial 4.8; r=sheehan

We need to run Mercurial 4.8 so we can start using SQLite storage
and wire protocol version 2 for partial clones.

--auto-deconfigure was added because desktop1604-test now inherits
from a base image with Mercurial installed.

Differential Revision: https://phabricator.services.mozilla.com/D11399