This moves away from the old method of creating a container with some shared volumes to one that most users would expect: any changes made in one step will be accessible by the following steps, and not only if they're in the workspace or `/etc/nix`. This also paves the way for a more generic Docker image engine, as users can do things like `apt install` without the results being blown away across steps. Signed-off-by: Winter <winter@winter.cafe>