Update geotiff directory permissions in unit tests

ci/test_mapserver_image.sh

@@ -124,7 +124,7 @@ EOF
 
 create_fake_geotiff() {
     bucket_name="g16_abi_radf_l1b_C01"
-    mkdir -p "${bucket_name}"
+    docker exec -i test bash -c "mkdir -p /data/${bucket_name}; chmod -R a+rwX /data/${bucket_name}"
     gtiff_fn="${bucket_name}/${C01_GTIFF_NAME}"
     docker exec -i test python3 <<EOF
 import rasterio