Require a minimum version of Dune for building

Author: Leonidas-from-XIV

bench/monorepo/bench.Dockerfile

@@ -135,7 +135,7 @@ RUN opam init --disable-sandboxing --auto-setup
 
 # make an opam switch for running benchmarks
 RUN opam update && opam switch create bench 4.14.2
-RUN opam install -y dune ocamlbuild
+RUN opam install -y 'dune>=3.18' ocamlbuild
 
 # make an opam switch for preparing the files for the benchmark
 RUN opam switch create prepare 4.14.2