Revert "Dockerfile.deploy: Don't VOLUME /var/lib/docker"

This reverts commit cddeebf6.

Now I know what the purpose of VOLUME /var/lib/docker was. Without
this volume, performing an incremental image build in the deploy
container takes a very long time (> 6 minutes) even though only one or
two files changed. The operation should take about 30 seconds.

I'll have to figure out a way to make sure that leaked volumes are
easily cleaned up by ./train-dev trim-images.