Support a never-pull policy for locally built images

[gologames]

I test locally built images before publishing them. When an image is missing locally, I want startup to fail instead of pulling from a registry.

Returning false from shouldPull() still allows pulling a missing image. Could you add an equivalent to docker run --pull=never?

I can submit a PR if you approve the proposal.

[cristianrgreco]

Makes sense, PR welcome! Probably a new, exported NeverPullPolicy or something

[gologames]

@cristianrgreco Opened #1460 to implement this