Amend the comment about the size of the docker image

Dockerfile.dev

@@ -24,6 +24,8 @@ RUN ./build-llvm.sh $TOOLCHAIN_PREFIX
 # Strip the LLVM install output immediately. (This doesn't reduce the
 # total docker image size as long as it is in a separate RUN layer though,
 # but reduces build times if tweaking the contents of strip-llvm.sh.)
+# Most of the size of the docker image comes from the build directory that
+# we keep in any case.
 COPY strip-llvm.sh .
 RUN ./strip-llvm.sh $TOOLCHAIN_PREFIX