Sets entrypoint-service for rock (#6)

The NFS helm chart passes additional arguments to the container image,
which should be passed to the binary itself. For this, we need to set
the entrypoint-service, so Pebble will pass the arguments to the proper
service.

nfsplugin/4.7.0/rockcraft.yaml

@@ -25,10 +25,12 @@ services:
   nfsplugin:
     override: replace
     startup: enabled
-    command: "/nfsplugin"
+    command: "/nfsplugin [ --help ]"
     on-success: shutdown
     on-failure: shutdown
 
+entrypoint-service: nfsplugin
+
 parts:
   add-dependencies:
     plugin: nil