Option for using the local k3d registry as a proxy for the Docker Hub

[inercia]

With this option, the local k3d registry can act as a pull-through cache for the Docker hub.

For example, after creating the cluster with:

k3d create --enable-registry --enable-registry-cache --registry-volume=k3d-registry

we can see that the registry is caching the images:

$ docker exec -ti k3d-registry ls -lisa /var/lib/registry/docker/registry/v2/repositories/library /var/lib/registry/docker/registry/v2/repositories/rancher
/var/lib/registry/docker/registry/v2/repositories/library:
total 16
42206594      4 drwxr-xr-x    4 root     root          4096 Mar 13 09:52 .
33554483      4 drwxr-xr-x    5 root     root          4096 Mar 13 09:52 ..
42860646      4 drwxr-xr-x    5 root     root          4096 Mar 13 09:52 busybox
42206595      4 drwxr-xr-x    5 root     root          4096 Mar 13 09:52 traefik

/var/lib/registry/docker/registry/v2/repositories/rancher:
total 28
33554484      4 drwxr-xr-x    7 root     root          4096 Mar 13 09:52 .
33554483      4 drwxr-xr-x    5 root     root          4096 Mar 13 09:52 ..
33554829      4 drwxr-xr-x    5 root     root          4096 Mar 13 09:52 klipper-helm
33554475      4 drwxr-xr-x    5 root     root          4096 Mar 13 09:52 klipper-lb
33554819      4 drwxr-xr-x    5 root     root          4096 Mar 13 09:52 local-path-provisioner
33554850      4 drwxr-xr-x    5 root     root          4096 Mar 13 09:52 metrics-server
33554485      4 drwxr-xr-x    5 root     root          4096 Mar 13 09:52 pause

[iwilltry42]

LGTM!
Very cool feature, thank you!

[deiwin]

This cannot be used together with pushing local images to the registry. See https://stackoverflow.com/a/33616926/1456578 and https://docs.docker.com/registry/configuration/#proxy.

Pushing to a registry configured as a pull through cache is currently unsupported

One would just get unsupported as a response when trying to push to the registry.

Maybe the cache should be configured as a separate registry if enabled?

[iwilltry42]

Thanks for the hint @deiwin 👍
For now, we added documentation/hints regarding this (commit).

I agree that it makes sense to split this up into two separate registry containers in the future 👍

[svenhofstede-zz]

Does this only work for images pulled from Docker hub or also for images pull from private repositories?

[iwilltry42]

@svenhofstede with the mirrors statement in the registries.yaml you can redirect pulls for every registry to a different registry. So yes, the concept should as well work for any other registry, but this PR/flag only sets it up for DockerHub.
You can edit the file manually: https://github.com/rancher/k3d/blob/master/docs/registries.md#registries-file