Proxy with mirror config

We are looking to implement this. Is it possible to run it so it can be used in engine mirror configs. E.g. containerd:

```
[plugins.cri.registry]
  [plugins.cri.registry.mirrors]
    [plugins.cri.registry.mirrors."*"]
      endpoint = ["https://docker-registry.somewhere.com"]
```

We want to avoid having to set `http_proxy=` env var on the whole process.