fix: set TMPDIR for skopeo to writable location

docker-build-nix/action.yaml

@@ -87,6 +87,10 @@ runs:
         TARGET_IMAGE="docker://${{ inputs.registry }}/${{ inputs.image-name }}:${{ inputs.image-tag }}"
         echo "Pushing OCI image to: $TARGET_IMAGE"
 
+        # Set TMPDIR to a writable location
+        export TMPDIR="${TMPDIR:-$PWD/tmp}"
+        mkdir -p "$TMPDIR"
+
         # Build skopeo command with optional credentials
         SKOPEO_CMD="skopeo copy"
         if [ -n "${{ inputs.registry-username }}" ] && [ -n "${{ inputs.registry-password }}" ]; then