WIP: Add overlays repo on Windows

Author: dra27

src-opam/dockerfile_opam.ml

@@ -405,6 +405,10 @@ let separate_ocaml_compilers hub_id arch distro =
            if OV.Releases.is_dev ov then
              run "opam repo add beta git+https://github.com/ocaml/ocaml-beta-repository --set-default"
            else empty in
+         let add_overlay =
+           if os_family = `Windows then
+             run "opam repo add ocurrent-overlay git+https://github.com/ocurrent/opam-repository-mingw#overlay --set-default"
+           else empty in
          let default_switch_name = OV.(with_patch (with_variant ov None) None |> to_string) in
          let variants =
            empty @@@ List.map (create_switch ~arch distro) (OV.Opam.V2.switches arch ov)
@@ -421,6 +425,7 @@ let separate_ocaml_compilers hub_id arch distro =
            @@ run "opam init -k git -a /home/opam/opam-repository --bare%s"
                 (if os_family = `Windows then "--disable-sandboxing" else "")
            @@ add_remote
+           @@ add_overlay
            @@ variants
            @@ run "opam switch %s" default_switch_name
            @@ run "opam install -y depext%s"