Document how to load image to local cluster

[ericzzzzzzz]

Background

• some local kubernetes clusters cannot access images on local docker deamon directly, if the clusters want to use those images, those images are needed to load to the clusters.
• Skaffold does images loading for some clusters under the hood, like minikube(by using the docker inside minikube), Kind, K3D, but Skaffold are not doing image loading work for other local clusters like micro8s
• If images built by local docker daemon are not loaded to local clusters, deployment phase will fail, normally the uses will see images cannot be pull error
• As a workaround, skaffold users can config build hooks for their build to load images
• Note that the build hooks are able to access build output https://skaffold.dev/docs/pipeline-stages/lifecycle-hooks/#environment-variables
  example config for the workaround

apiVersion: skaffold/v4beta1
kind: Config
build:
  artifacts:
  - image: skaffold-example
    hooks:
      after:
        - command:
            - "bash"
            - "-c"
            - "xxx xxx xxx $SKAFFOLD_IMAGE"

• We should document this

[ericzzzzzzz]

Maybe skaffold should support creating a local registry with a container runtime? Then anything local cluster should just be able to pull images from the local registry, no more manual loading needed.