Taskcluster-taskgraph

Added

- Create a base docker image containing only run-task

Added

- Support for actions in pull-requests

Fixed

- Various fixes targeting tarfile reproducibility

Fixed

- Make run-task shebang use /usr/bin/env python3 (453)

Added

- Improved logging to optimization phase
- Utility function to walk through a task's dependency ancestors

Fixed

- `taskgraph init` support for github-release events in .taskcluster.yml
- `taskgraph init` .taskcluster.yml passes short refs to run-task
- Check to ensure reserved dependency edge `docker-image` isn't used
- Improved tracebacks when querying json-automationrelevance from hg.mozilla.org fails

Added

- Add a `get_ancestors` helper to find ancestor tasks through the Taskcluster API

Added

- Add a `cache-pull-requests` parameter to `taskcluster/config.yml` to disable caching of tasks on pull requests.