Install tidy on x86_64-gnu-aux target to run html check

Author: GuillaumeGomez

src/ci/docker/host-x86_64/x86_64-gnu-aux/Dockerfile

@@ -17,7 +17,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   libgl1-mesa-dev \
   llvm-dev \
   libfreetype6-dev \
-  libexpat1-dev
+  libexpat1-dev \
+  tidy
 
 COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh