Start up a demo autoland-transplant environment.

Edit `.env` to customise settings; it's likely you'd only need to adjust the
"exposed ports" settings.

Once the environment is running:

    1. Clone the repository with `clone-repo.py`.
       This will create a `test-repo` sub-directory.
    2. cd into `test-repo` and make changes.
    3. Use `hg` to commit your changes.
    4. Run `../post-to-autoland.py` to post draft commits to autoland.
    5. Run `hg pull` to update the commit's phase.