docker: install clang-12 to speedup the build jobs

Signed-off-by: John Crispin <john@phrozen.org>

docker/Dockerfile

@@ -6,6 +6,7 @@ RUN apt-get update \
             time git-core build-essential gcc-multilib clang \
             libncurses5-dev zlib1g-dev gawk flex gettext wget unzip python \
             python3 python3-pip python3-yaml libssl-dev rsync llvm llvm-12 \
+            clang-12 \
     && apt-get clean
 RUN git config --global user.email "you@example.com"
 RUN git config --global user.name "Your Name"