fix: specify image name in policy.json (blue-build#176)

There was talk on the discord about not being able to pull in images with podman because the signing policy included *every* image inside of the user's ghcr account. Which means that images not signed with the same key won't be able to be pulled down

config/scripts/signing.sh

@@ -11,7 +11,7 @@ cp /usr/share/ublue-os/cosign.pub /usr/etc/pki/containers/"$IMAGE_NAME".pub
 FILE=/usr/etc/containers/policy.json
 
 yq -i -o=j '.transports.docker |=
-    {"'"$IMAGE_REGISTRY"'": [
+    {"'"$IMAGE_REGISTRY"'/'"$IMAGE_NAME"'": [
             {
                 "type": "sigstoreSigned",
                 "keyPath": "/usr/etc/pki/containers/'"$IMAGE_NAME"'.pub",