docker: set default locale to C.UTF-8

Author: LCamel

docker/Dockerfile

@@ -9,4 +9,6 @@ RUN --mount=type=bind,source=install-haskell.sh,target=/tmp/install-haskell.sh \
     --mount=type=bind,source=haskell-versions.env,target=/tmp/haskell-versions.env \
     bash -x /tmp/install-haskell.sh
 
+ENV LANG=C.UTF-8
+
 CMD ["bash"]