ci: Add skopeo login to push_image script (#438)

Author: hneiva

taskcluster/docker/skopeo/push_image.sh

@@ -15,6 +15,8 @@ PASSWORD_URL="http://taskcluster/secrets/v1/secret/project/taskgraph/level-3/doc
 install -m 600 /dev/null $HOME/.dockercfg
 curl $PASSWORD_URL | jq '.secret.dockercfg' > $HOME/.dockercfg
 
+skopeo login docker.io
+
 cd $MOZ_FETCHES_DIR
 unzstd image.tar.zst