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

[jlowin]

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.

[cicdw]

Ideally we would auto-tag the image name with the flow ID itself, but there's a chicken / egg problem with that.

[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

[cicdw]

I love that.

[cicdw]

It looks like using .isoformat() or str(timestamp) violates the restrictions of docker tags, so we'll need to use something like slugify.slugify(str(pendulum.now("utc")))

[joshmeek]

Yeah I'm game for timestamp as default