Hide Mapnik warnings once available (5.1) (#884)

Uses the patch by @zstadler to filter out irrelevant Mapnik warnings.
This will be ignored until tools v5.1 is used.

.env

@@ -17,3 +17,5 @@ DIFF_MODE=false
 BBOX=-180.0,-85.0511,180.0,85.0511
 MIN_ZOOM=0
 MAX_ZOOM=14
+
+FILTER_MAPNIK_OUTPUT=1

docker-compose.yml

@@ -67,6 +67,7 @@ services:
       BBOX: ${BBOX}
       MIN_ZOOM: ${MIN_ZOOM}
       MAX_ZOOM: ${MAX_ZOOM}
+      FILTER_MAPNIK_OUTPUT: ${FILTER_MAPNIK_OUTPUT}
 
   postserve:
     image: "openmaptiles/openmaptiles-tools:${TOOLS_VERSION}"