Overwrite registry image ENTRYPOINT to avoid panic when scheduler-state.json file is corrupted.

[dimitar-kostadinov]

How to categorize this PR?

/area robustness
/kind enhancement

What this PR does / why we need it:
Implements mitigation for distribution/distribution#4478.
The registry-cache container is extended with command that checks scheduler-state.json and if it is corrupted, to clean up the persistent volume mounted under /var/lib/registry.
The command that perform the check and cleanup is:

      repoRoot=/var/lib/registry
      if [ -f "${repoRoot}/scheduler-state.json" ]; then
          if [ -s "${repoRoot}/scheduler-state.json" ]; then
              echo "The scheduler-state.json file is OK"
          else
              echo "Cleanup corrupted scheduler-state.json file"
              rm -f "${repoRoot}/scheduler-state.json"
              echo "Cleanup docker directory"
              rm -rf "${repoRoot}/docker"
          fi
      else
          echo "The scheduler-state.json file is not created yet"
      fi

      source /entrypoint.sh /etc/distribution/config.yml

Which issue(s) this PR fixes:
Fixes #291

Special notes for your reviewer:
Steps to reproduce the issue:

1. Exec to registry container, e.g.k -n kube-system exec -it registry-docker-io-0 -- sh, and corrupt the file:

   rm var/lib/registry/scheduler-state.json
   touch var/lib/registry/scheduler-state.json
   

2. Exec to the node, e.g. k -n shoot--local--local exec -it machine-shoot--local--local-local-6cffc-lxvbn -- bash, and kill the registry container:

   ctr -n k8s.io task kill -s SIGKILL c16425e8b798203b9d011b6445b0a2c483ae74b13f98f1c7c415c3868d240250
   

   use containerId from registry pod: status.containerStatuses[0].containerID.

Release note:

The following Distribution issue [distribution/distribution#4478](https://github.com/distribution/distribution/issues/4478) is now mitigated.

[dimitar-kostadinov]

Known flake: #290

/test pull-gardener-extension-registry-cache-e2e-kind

/test pull-gardener-extension-registry-cache-unit

[ialidzhikov]

/assign

[dimitar-kostadinov]

/hold

[dimitar-kostadinov]

/unhold

[ialidzhikov]

/lgtm

[gardener-prow]

LGTM label has been added.

Git tree hash: a32d790a8fcfaa7e359d572635d9114af3b40b87

[gardener-prow]

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