ci: Fix pod UIDs/GIDs

Now that we have _democratic-csi_ for storage management, the old manual
iSCSI volumes are being replaced with dynamically provisioned volumes.
ThiThe new _buildroot-airplaypi_ volume is completely blank, so _root_
owns everything.  The old volume had the correct ownership because it
was originally mounted in a pod that had the default `securityContext`,
before we changed the merge strategy.  We now need to explicitly set the
UIDs and GIDs, since we're not inheriting the default `securityContext`
anymore.

ci/podTemplate.yaml

@@ -25,6 +25,9 @@ spec:
   nodeSelector:
     kubernetes.io/arch: amd64
   securityContext:
+    runAsUser: 1000
+    runAsGroup: 1000
+    fsGroup: 1000
     fsGroupChangePolicy: OnRootMismatch
     seLinuxOptions:
       level: s0:c596,c675