Bump OCaml version to 4.09 (and remove spurious opam install dune)

Dockerfile

-FROM ocaml/opam2:4.07
+FROM ocaml/opam2:4.09
 COPY --chown=opam id_rsa /home/opam/.ssh/
 RUN echo gitlab.inria.fr,128.93.193.8 ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBAFldY2ft1ReZjFYPpe/wa5Vhl4YJfYz6IJOZaZxL924R44jrGWiR2/Misrug75NBsQB+UWs1iIyWZPk0AN45Sk= >/home/opam/.ssh/known_hosts
 RUN git pull
@@ -6,7 +6,6 @@ RUN opam update
 RUN opam pin add -n git+https://gitlab.inria.fr/tmartine/clangml.git#snapshot
 RUN opam depext -i clangml
 RUN git clone git@gitlab.inria.fr:tmartine/clangml-transforms.git
-RUN opam install dune
 RUN cd clangml-transforms && dune build clangml-transforms.opam
 RUN opam pin add -n file://$PWD/clangml-transforms
 RUN cd /home/opam && git clone git@gitlab.inria.fr:tmartine/memcad.git