Disable blob caching when garbage-collection is enabled

The garbage-collection cronjob will cause the registry to go into
an inconsistent state if the blob cache is not cleared afterward.

Since the cronjob has no facility for doing that, disable blob
caching entirely when garbage-collection is in use.

Fixes: Issue #104

templates/configmap.yaml

@@ -1,3 +1,7 @@
+{{- /* Blob caching and the garbage-collection cronjob do not mix */ -}}
+{{- if .Values.garbageCollect.enabled -}}
+{{- $_ := set .Values.configData.storage.cache "blobdescriptor" "disabled" -}}
+{{- end -}}
 apiVersion: v1
 kind: ConfigMap
 metadata: