[FEATURE] support for pull-through registry

[ethinx]

Is your feature request related to a problem or a Pull Request

Hi k3d team,

This is a feature request about registry sub-command.

I tried k3d recently and found k3d could create and manage registry node, connect the clusters to registries automatically, it's great.

But everytime I rebuild the cluster, k3d need to pull the images of cluster from upstream, due to poor network conditions, it takes a long time to be ready.

I tried to find if there is a way to make local cache with the k3d managed registry and configure the cluster pull from the cache, it seems that k3d could only create a simple registry now, but not a pull-through cache.

Scope of your request

New flags for k3d registry create sub-commands

Describe the solution you'd like

I am thinking that if we could introduce new flags for k3d registry create to create a pull-through registry.

Describe alternatives you've considered

k3d registry create \
  --proxy-remote-url <remote_url> \
  --proxy_username <username> \
  --proxy_password <password> \
  -v <src_path>:<dst_path> \
  ...

Demo:

1. Create a pull-through registry

k3d registry create docker-io \
  -p 127.0.0.1:5000 \
  --proxy-remote-url https://registry-1.docker.io \
  -v ~/.local/share/docker-io-registry:/var/lib/registry

2. Create registry.yaml

mirrors:
  "docker.io":
    endpoint:
      - http://k3d-docker-io:5000

3. Create cluster and use the pull-through cache

k3d cluster create cluster01 --registry-use k3d-docker-io:5000 --registry-config registry.yml

4. After the cluster01 ready, create another cluster with the same registry or rebuild the cluster, it would be fast.

k3d cluster create cluster02 --registry-use k3d-docker-io:5000 --registry-config registry.yml

I see there are already options about registry proxy but they are not used in the registry creation.

I made a fork(ethinx@1160b23) about the above idea, it's a draft and please take a look if it's appropriate.

[iwilltry42]

Hi @ethinx , thanks for opening this issue!
Please feel free to open a PR using your fork and I'll happily review and test it 👍

[ethinx]

Thanks for your time @iwilltry42, just create a pr for this issue, please let me know if there is any problem with the pr.