ci: fix workflow YAML quoting for multi-line image list

Author: jmewing

.github/workflows/docker-publish.yml

@@ -49,11 +49,10 @@ jobs:
         run: |
           IMAGES="${{ env.GHCR_IMAGE }}"
           if [ -n "${DOCKERHUB_USERNAME}" ]; then
-            IMAGES="${IMAGES}
-${DOCKERHUB_USERNAME}/lux-mon"
+            IMAGES="${IMAGES} ${DOCKERHUB_USERNAME}/lux-mon"
           fi
           echo "list<<EOF" >> "$GITHUB_OUTPUT"
-          echo "$IMAGES" >> "$GITHUB_OUTPUT"
+          for img in ${IMAGES}; do echo "${img}" >> "$GITHUB_OUTPUT"; done
           echo "EOF" >> "$GITHUB_OUTPUT"
 
       - name: Set up QEMU