avoid docker engine check when unnecessary

[huanjani]

If users are bringing their own built image, they should not have to have their docker engine running.

Currently, they can use the --image flag or follow the prompt for image triggered by no docker engine detected.

However, in the case that a user has an existing manifest with the image field populated, they can't get past our prompts and errors about docker engines.

If they don't use the --image flag and then hit 'Enter' when prompted for it, we should trigger RequireNonEmpty() instead of erroring out about the docker engine.

If we find an existing manifest, we should parse the image value out of it, as we do for healthcheck.

Related: #3639