Allow a spawn request which does not pull the image

[paulgb]

This is something I've wanted to do to reduce cold start times when we know we have the image locally, and has also been requested by @MyCADDev via Discord.

[pretentious7]

I can do this tom! Should just be matter of putting the pull behind a conditional right? And adding a field to the request.

[paulgb]

I think we should support three pull policies like Kubernetes does:

• IfNotPresent should be the default. It pulls only if we don't already have the image. We would switch to this for our own use case.
• Always is the current behavior. Always run a Docker pull, even if the image does not exist.
• Never never pull the image. If it doesn't exist, the backend status should be ErrorLoading. This would be the pull policy that @MyCADDev would use.

This would become a field on the DockerExecutableConfig rather than the SpawnRequest itself.

[pretentious7]

Oops have to add this to the cli before closing.

[MyCADDev]

Just tried it via NATS API - works like a charm. Thanks @paulgb and @pretentious7

[paulgb]

@pretentious7 I think it's not necessary for this to be in the cli, but we can discuss if you want to. I'll close this for now because the feature is done.