fix(deployment): run plugin container as the reposilite user

The reposilite entrypoint chowns /app to 977:977 whenever it starts as root. The plugin container downloaded
the jar as its own image user (101), so after the first start the file was owned by 977:977 with mode 0644.
Since the plugins emptyDir survives a container restart within the same pod, a restarted plugin container
could no longer overwrite the existing jar and aborted with "Permission denied", which left the pod in a
permanent Init:CrashLoopBackOff until the pod itself was recreated.

Run the plugin container as 977:977 by default and expose the security context as
deployment.pluginContainer.securityContext, so it can be aligned when PUID/PGID are overridden.

Co-authored-by: Copilot <copilot@github.com>

unittests/deployment/configPlugins.yaml

@@ -30,6 +30,9 @@ tests:
         - https://reposilite.com/plugins/prometheus.jar
         name: download-prometheus-plugin
         image: docker.io/curlimages/curl:0.1.0
+        securityContext:
+          runAsGroup: 977
+          runAsUser: 977
         volumeMounts:
         - mountPath: /app/data/plugins
           name: plugins