Add a bootc.diskimage-builder label

Right now, the relationship between this image and bootc-image-builder
is "floating".  However, we may want the ability to more
strictly bind the two in the future.

With this, the bootc image links out to the recommended diskimage
builder container, so automated tooling can find it instead
of hardcoding it.

Signed-off-by: Colin Walters <walters@verbum.org>

centos-bootc-config.json

@@ -1,6 +1,7 @@
 {
     "Labels": {
         "containers.bootc": "1",
+        "bootc.diskimage-builder": "quay.io/centos-bootc/bootc-image-builder",
         "redhat.compose-id": "CentOS-Stream-9-20240304.d.0",
         "redhat.id": "centos",
         "redhat.version-id": "9"