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