ci: Only write docker build images to Cirrus cache

Other cache providers offer too little space for this to be useful.

.github/actions/configure-docker/action.yml

@@ -48,8 +48,8 @@ runs:
         # Always optimistically --cache‑from in case a cache blob exists
         args=(--cache-from "type=gha${url_args:+,${url_args}},scope=${CONTAINER_NAME}")
 
-        # If this is a push to the default branch, also add --cache‑to to save the cache
-        if [[ ${{ github.event_name }} == "push" && ${{ github.ref_name }} == ${{ github.event.repository.default_branch }} ]]; then
+        # Only add --cache-to when using the Cirrus cache provider and pushing to the default branch.
+        if [[ ${{ inputs.cache-provider }} == 'cirrus' && ${{ github.event_name }} == "push" && ${{ github.ref_name }} == ${{ github.event.repository.default_branch }} ]]; then
           args+=(--cache-to "type=gha${url_args:+,${url_args}},mode=max,ignore-error=true,scope=${CONTAINER_NAME}")
         fi