Bump version of docker registry

See if this helps with https://jupyter.zulipchat.com/#narrow/channel/469744-jupyterhub/topic/Hetzner.20push.20error/with/505498241

Author: yuvipanda

mybinder/templates/registry/deployment.yaml

@@ -42,7 +42,7 @@ spec:
       {{- end }}
       containers:
       - name: registry
-        image: registry:3.0.0-rc.2
+        image: registry:3.0.0-rc.3
         volumeMounts:
           - name: registry-config
             # This path is what registry documentation *says* we should put