Add a containers.bootc label

Pairs with https://github.com/containers/bootc/pull/299

This is intended to be the new canonical label for bootc-compatible
images, as opposed to the existing `ostree.linux` etc.

centos-bootc-config.json

@@ -1,5 +1,6 @@
 {
     "Labels": {
+        "containers.bootc": "1",
         "redhat.compose-id": "CentOS-Stream-9-20240205.d.0",
         "redhat.id": "centos",
         "redhat.version-id": "9"

fedora-bootc-config.json

@@ -1,5 +1,6 @@
 {
     "Labels": {
+        "containers.bootc": "1",
         "redhat.compose-id": "Fedora-ELN-20240205.2",
         "redhat.id": "fedora",
         "redhat.version-id": "ELN"