Updates the Docker configuration to use the gunicorn server with gevent
workers by default. Adds `waitress` to the docker container, so that if
no server is specified, we will fall back to that rather than `wsgiref`.
Making this happen brought a few other issues to light, which are also
addressed here.
- Docker log output not immediately being flushed to stdout (#358):
resolved by setting the `PYTHONUNBUFFERED` env var to `t` in the
docker container
- When the WSGIRef server is selected, its access logs are written
directly to stderr, rather than going through the logging machinery:
resolved by adding a new `WsgiHandler` class and passing in to
bottle's `run()` method when running the wsgi server. This required a
new `ServerCheck` class to determine whether the wsgi server is
selected when the `auto` option is used
- When using `gunicorn` along with the watchdog cache, package uplaods
were not being picked up by the watcher. Updated the `add_package`
and `remove_package` methods on the `CachingFileBackend` to bust the
cache
