Allow most tasks to use a docker container

[asmeurer]

Most tasks should be able to use a Docker container, similar to the pytest activity. The exception would be the activities that push things up like push_tags and pypi, since the container won't have access to the credentials to push.

Another thing that would be useful is a similar option but that uses a temporary directory and a virtualenv/conda env, as something more lightweight.

[asmeurer]

Also, conversely, the pytest activity should have the option to not use Docker.