Custom Docker registry / user

This adds support for a custom Docker user and / or registry by changing config in `tup.config`, e.g.:

```
CONFIG_DOCKER_USER=lmars
CONFIG_DOCKER_REGISTRY=my.registry.com
```

If the registry needs basic auth, that can be set in the URI:

```
CONFIG_DOCKER_REGISRTY=user:password@my.registry.com
```

Note: pushing to the private registry will require a `docker login my.registry.com` first as `docker push` doesn't accept basic auth in the URI.

Closes #270.