config: Add ENV container=oci

xref https://bugzilla.redhat.com/show_bug.cgi?id=2278652

I think this is a best practice for base images;

fedora-bootc-config.json

@@ -5,5 +5,8 @@
         "redhat.id": "fedora",
         "redhat.version-id": "40"
     },
-    "StopSignal": "SIGRTMIN+3"
+    "StopSignal": "SIGRTMIN+3",
+    "Env": [
+        "container=oci"
+    ]
 }