Document minikube image semantics about locations

[afbjorklund]

minikube image, an alternative to using minikube docker-env and docker image that supports multiple container runtimes

(originally we added minikube podman-env and podman-remote image, but it didn't scale to other runtimes - like containerd)

Image Command	CRI	Comment
build	No (used to be in CBI)	Dockerfile + build context
load	No	load image from tarball
save	No	save image to tarball
ls	Yes (crictl images)
pull	Yes (crictl pull)	pull image from remote
push	No	push image to remote
rm	Yes (crictl rmi)
tag	No

load

a)
This command loads an image to the minikube cluster

minikube image load image

1. Load image from docker cache, ~/.minikube/cache/images
2. Save image from local docker daemon (--daemon), to cache
3. Pull image from remote docker registry (--remote), to cache

Note: the image will always been copied into the cache directory

b)
Alternatively, load image directly from local tarball file (.tar)

minikube image load image.tar

Note: the image will not be copied into the cache/daemon/remote

pull

Not implemented (separately), see minikube image load --pull.

minikube image pull image

save

a)
This commands saves an image from the minikube cluster

minikube image save image

1. Save image to docker cache, ~/.minikube/cache/images
2. Load image to local docker daemon (--daemon), from cache
3. Push image to remote docker registry (--remote), from cache

Note: the image will always been copied into the cache directory

b)
Alternatively, save image directly to local tarball file (.tar)

minikube image save image.tar

Note: the image will not be copied into the cache/daemon/remote

push

Not implemented (separately), see minikube image build --push.

minikube image push image

---

In the current implementation, "load" will automatically check with daemon and remote if the image is not found in the cache.
This means that you might have to use --daemon=false --remote=false if you want to avoid this, e.g. no Docker or if offline

In the current implementation, "save" will default to the cache and only upload to daemon and remote if explicitly requested to.
This means that you might have to use --daemon=true --remote=true if you expect this uploading to happen automatically

Note: currently we are using crane to do the downloading.

See https://github.com/google/go-containerregistry

It will use the "tarball" format by default, not "legacy".

This package produces tarballs that can consumed via docker load. Note that this is a different format from the legacy tarballs that are produced by docker save, but this package is still able to read the legacy tarballs produced by docker save.

Currently there is no minikube support for OCI images.

[afbjorklund]

Note: minikube cache add is the same as minikube image load, but also records the image in the configuration...

This means that the image will be loaded by default, for all new clusters. View the config with minikube cache list

[afbjorklund]

Note: expecting a Docker daemon to always be running locally is legacy behavior, and might be disabled in the future

See: #11059

Note: the cache directory might be replaced by a Distribution server in the future, which would also allow push and pull

See: #12087

[afbjorklund]

Need to implement pull and tag as well, if nothing else then for testing the save command.

PR #12265

Might throw in push for completeness, should be very similar to tag. Maybe it will be useful.

See: #12323