Create fontconfig cache dir in Docker image

docker/Dockerfile

@@ -36,7 +36,10 @@ ENV VIPS_WARNING=0
 ENV MALLOC_ARENA_MAX=2
 ENV LD_LIBRARY_PATH /usr/local/lib
 
-RUN groupadd -r imgproxy && useradd -r -u 999 -g imgproxy imgproxy
+RUN groupadd -r imgproxy \
+  && useradd -r -u 999 -g imgproxy imgproxy \
+  && mkdir -p /var/cache/fontconfig \
+  && chmod 777 /var/cache/fontconfig
 USER 999
 
 CMD ["imgproxy"]