The base docker stage shouldn't copy app

[dannystaple]

This means that too many other stages rebuild and busts the cache unnecessarily.

Move the App copy out of the base docker stage and to the last possible moment of the stages that need it.

acceptance criteria

• the app code should not be copied at the base stage
• the final unnamed stage should have the app code copied in it.
• tests should still run correctly
• the site should still have built correctly
• all editor config conventions to be followed