Bug 1492664 - set TASKCLUSTER_ROOT_URL and TASKCLUSTER_PROXY_URL; r=tomprince,glandium

Eventually, workers will provide these variables directly
(https://bugzilla.mozilla.org/show_bug.cgi?id=1460015). But for now, this
ensures that TASKCLUSTER_ROOT_URL is set everywhere, and TASKCLUSTER_PROXY_URL
is set wherever the proxy is active.

The setup for the mach commands defaults to https://taskcluster.net for user
convenience. When the production instance's URL changes, we can simply change
that default.

This changes the docker build process to propagate TASKCLUSTER_ROOT_URL into
the docker images, and for good measure includes some code to use that value to
generate debian repo paths.

Differential Revision: https://phabricator.services.mozilla.com/D14196