ci: fix push-image tasks to use the right dependency (#2765)

Using "docker-image" as the key in the dependencies dict clashes with
what taskgraph uses for docker-worker in-tree images, so we ended up
fetching the wrong image.  Use a different key to resolve this conflict.

taskcluster/ci/push-image/kind.yml

@@ -29,7 +29,7 @@ task-defaults:
         command:
             - /usr/local/bin/push_image.sh
     fetches:
-        docker-image:
+        image:
             - artifact: image.tar.zst
               extract: false
     scopes:
@@ -39,15 +39,15 @@ tasks:
     balrog-backend:
         description: "Push balrog backend docker image"
         dependencies:
-              docker-image: build-docker-image-balrog-backend
+              image: build-docker-image-balrog-backend
         worker:
             env:
                 DOCKER_REPO: docker.io/mozilla/balrog
                 APP: backend
     balrog-agent:
         description: "Push balrogagent docker image"
         dependencies:
-              docker-image: build-docker-image-balrog-agent
+              image: build-docker-image-balrog-agent
         worker:
             env:
                 DOCKER_REPO: docker.io/mozilla/balrogagent