Use utf8 when building docs
Update to docker-py 0.3.0

From https://github.com/dotcloud/docker-py/commit/375521d7e2f4ec40d86945aed0b7f5eb31f889ed

This is mostly an upgrade to v1.8 of the remote API.
Fix build output

docker-py now streams us the raw JSON events, so we have to replicate
the Docker client's progress logic.

On the bright side, we now have well-behaved progress bars when pulling
an image during `fig build` (no more ski slopes) and `fig up` (no more
silence).