Add apt clean to save some disk space.

Dockerfile-ubuntu2004

@@ -52,6 +52,8 @@ COPY switch-to-latest-clang /usr/local/bin/switch-to-latest-clang
 
 RUN chmod +x /usr/local/bin/switch-to-latest-clang
 
+RUN apt clean
+
 FROM scratch
 
 COPY --from=build1 / /