Add VOLUME /var into container config by default

This meshes with
<https://github.com/ostreedev/ostree/pull/3166/commits/f81b9fa1666c62a024d5ca0bbe876321f72529c7>
in that our container image now behaves by default
similarly to how the host works at runtime.

centos-bootc-config.json

@@ -4,5 +4,8 @@
         "redhat.compose-id": "CentOS-Stream-9-20240212.d.0",
         "redhat.id": "centos",
         "redhat.version-id": "9"
+    },
+    "Volumes": {
+        "/var": {}
     }
 }

fedora-bootc-config.json

@@ -4,5 +4,8 @@
         "redhat.compose-id": "Fedora-ELN-20240213.3",
         "redhat.id": "fedora",
         "redhat.version-id": "ELN"
+    },
+    "Volumes": {
+        "/var": {}
     }
 }