postgres: run unprivilged

Change-Id: I8d7e92093c0df91b6cd601a4d8e2484fca97ee88
diff --git a/kube/postgres.libsonnet b/kube/postgres.libsonnet
index d82a89a..571fe7b 100644
--- a/kube/postgres.libsonnet
+++ b/kube/postgres.libsonnet
@@ -67,6 +67,9 @@
                             },
                         },
                     },
+                    securityContext: {
+                        runAsUser: 999,
+                    },
                 },
             },
         },