Fix ECR authentication failure

Skopeo 1.11 changed behavior: when an `--authfile` flag is present, read
only those creds. We weren't settingthat flag ourselves but the skopeo
container does?

.buildkite/steps/publish-docker-image.sh

@@ -15,7 +15,11 @@ dry_run() {
 }
 
 skopeo() {
-  docker run --rm -v "${DOCKER_CONFIG:-$HOME/.docker}:/root/.docker:ro" quay.io/skopeo/stable:v1 "$@"
+  docker run --rm \
+    -v "${DOCKER_CONFIG:-$HOME/.docker}:/root/.docker:ro" \
+    -e "REGISTRY_AUTH_FILE=/root/.docker/config.json" \
+    quay.io/skopeo/stable:v1 \
+    "$@"
 }
 
 # Convert 2.3.2 into [ 2.3.2 2.3 2 ] or 3.0-beta.42 in [ 3.0-beta.42 3.0 3 ]