Apply suggestion from @MisterDA

use `ADD` to download HTML compiler manuals repo

Co-authored-by: Antonin Décimo <antonin.decimo@gmail.com>

Author: sabine

Dockerfile

@@ -42,7 +42,7 @@ COPY --from=build --link "/root/ocaml.org/_build/default/src/ocamlorg_web/bin/ma
 
 COPY --link playground/asset playground/asset
 
-RUN git clone https://github.com/ocaml-web/html-compiler-manuals /manual
+ADD --keep-git-dir --link https://github.com/ocaml-web/html-compiler-manuals /manual
 ADD --link data/v2 /v2
 
 RUN git config --global --add safe.directory /var/opam-repository