build(docker/frontend): add locale env vars to stop perl from complaining

docker/frontend/Dockerfile

@@ -4,6 +4,8 @@ ARG FROM_TAG=20
 FROM ${FROM_IMG}:${FROM_TAG}
 
 ENV LANG=de_DE.UTF-8
+ENV LANGUAGE=de_DE.UTF-8
+ENV LC_ALL=de_DE.UTF-8
 
 # build and watch dependencies
 RUN apt-get -y update && apt-get -y install exiftool