Docker: only write to cache if creds available

.github/scripts/docker-build.bash

@@ -1,6 +1,19 @@
 #!/usr/bin/env bash
 set -ex
 
+# Only build amd64 on PRs, build all platforms on main. The arm builds
+# take far too long.
+image_platforms="--platform linux/amd64"
+push_image=""
+cache_tag="pr-cache"
+
+# 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
 
@@ -24,12 +37,6 @@ if [[ "$RELEASE" == ON ]]; then
     image_tags+=("--tag" "$DOCKERHUB_IMAGE_ID:latest")
 fi
 
-# Only build amd64 on PRs, build all platforms on main. The arm builds
-# take far too long.
-image_platforms="--platform linux/amd64"
-push_image=""
-cache_tag="pr-cache"
-
 # Only push on main
 if [[ "$GITHUB_REF" == refs/heads/main ]]; then
     push_image="--push"
@@ -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[@]}" \
     .