title: Unable to push image to a docker registry configured as proxy cache
body: <p>I followed <a href="http://blog.docker.com/2015/10/registry-proxy-cache-docker-open-source/" rel="noreferrer">this</a> guide to setup a Docker v2 Registry acting as a local proxy cache for Docker Hub images. My Docker daemon is configured with both <code>--insecure-registry</code> and <code>--registry-mirror</code> options pointing to the same registry instance.</p>
  
  <p>When pulling images it works correctly by caching them to the local store.</p>
  
  <p>The problem is that when I try to push an image to such local private registry, I get a weird <code>UNSUPPORTED</code> error. The registry log says:</p>
  
  <pre><code>time="2015-11-09T13:20:22Z" level=error msg="response completed with error" err.code=UNSUPPORTED err.message="The operation is unsupported." go.version=go1.4.3 http.request.host="my.registry.io:5000" http.request.id=b1faccb3-f592-4790-bbba-00ebb3a3bfc1 http.request.method=POST http.request.remoteaddr="192.168.0.4:57608" http.request.uri="/v2/mygroup/myimage/blobs/uploads/" http.request.useragent="docker/1.9.0 go/go1.4.2 git-commit/76d6bc9 kernel/3.16.0-4-amd64 os/linux arch/amd64" http.response.contenttype="application/json; charset=utf-8" http.response.duration=2.035918ms http.response.status=405 http.response.written=78 instance.id=79970ec3-c38e-4ebf-9e83-c3890668b122 vars.name="mygroup/myimage" version=v2.2.0
  </code></pre>
  
  <p>If I disable proxy setting on the registry then the push works correctly. Am I missing something on the configuration or it is just that a private registry cannot act as a proxy cache at the same time?</p>