investigate alternate solutions for the taskcluster directory

[escapewindow]

investigate alternate solutions for the taskcluster directory

[tomprince]

* using `--root` implies the source is in the xpi-template directory, which breaks

I haven't looked at all the test failures, but the one I looked it, it appears that various bits of the docker support code assumes that docker images are in the current directory, under taskcluster/docker. This did not affect comm-central, since it was re-using the m-c images, so that was correct. This is definitely something we can address, by passing down the correct directory to taskgraph.util.docker in various places.

* we could also run in the xpi-template directory (with or without `--root`), and specify a known location for the xpi source.

This is how ci-config works with ci-admin (ci-config is checked out in config/).

  * I believe this will reuse the level from the xpi-template repo instead of the source repo

The level should come entirely from the arguments to taskgraph decision, which comes from .taskgraph.yml.