Make default image tag `latest` (or None) for Docker storage

Right now the image name and image tag are both random UUIDs. While this does mean that collisions are on the order of (1 / the atoms in the universe) / the atoms in the universe, having a random tag is probably redundant.

[jlowin]

Proposal:

image:tag -> {flow.name}:{timestamp}

If, in addition, we updated it with {flow.name}:latest as well, then users could trivially pull and test new builds

[joshmeek]

Yeah I’m game for timestamp as default