Docker: only write to cache if creds available

.github/scripts/docker-build.bash

@@ -1,6 +1,13 @@
 #!/usr/bin/env bash
 set -ex
 
+# Only write to cache if credentials are available
+if [[ -z "$DOCKERHUB_USERNAME" ]]; then
+    write_cache=""
+else
+    write_cache="--cache-to=type=registry,ref=$DOCKERHUB_USERNAME/$IMAGE_NAME:$cache_tag,mode=max"
+fi
+
 DOCKERHUB_USERNAME="${DOCKERHUB_USERNAME:-conky}"
 DOCKERHUB_IMAGE_ID=$DOCKERHUB_USERNAME/$IMAGE_NAME
 
@@ -41,6 +48,6 @@ docker buildx build \
     ${push_image} \
     ${image_platforms} \
     --cache-from=type=registry,ref=$DOCKERHUB_USERNAME/$IMAGE_NAME:$cache_tag \
-    --cache-to=type=registry,ref=$DOCKERHUB_USERNAME/$IMAGE_NAME:$cache_tag,mode=max \
+    ${write_cache} \
     "${image_tags[@]}" \
     .