Split MSVC and Cygwin install into a separate build step

Author: MisterDA

src-opam/opam.ml

@@ -480,38 +480,37 @@ let windows_msvc_opam2 ?win10_revision ?winget ?(labels = []) ~opam_hashes
   let winget_image, winget_setup =
     install_winget ?win10_revision ?winget version
   in
-  let packages, vs_build_tools =
-    ( Windows.Cygwin.msvc_packages,
-      Windows.install_visual_studio_build_tools
-        [
-          "Microsoft.VisualStudio.Component.VC.Tools.x86.x64";
-          (* Without 18362, rc.exe is missing from the Path. *)
-          "Microsoft.VisualStudio.Component.Windows10SDK.18362";
-        ] )
-  in
-  let opams_image =
-    Windows.header ~alias:"opam-builder" ?win10_revision ~version ()
+  let cygwin_msvc_image =
+    let packages, vs_build_tools =
+      ( Windows.Cygwin.msvc_packages,
+        Windows.install_visual_studio_build_tools
+          [
+            "Microsoft.VisualStudio.Component.VC.Tools.x86.x64";
+            (* Without 18362, rc.exe is missing from the Path. *)
+            "Microsoft.VisualStudio.Component.Windows10SDK.18362";
+          ] )
+    in
+    Windows.header ~alias:"cygwin-msvc" ?win10_revision ~version ()
     @@ Windows.sanitize_reg_path ()
     @@ vs_build_tools
     @@ Windows.Cygwin.install_cygwin ~msvs_tools:true ~extra:packages ()
+  in
+  let opams_image =
+    Dockerfile.from ~alias:"opam-builder" "cygwin-msvc"
     @@ install_opams_windows ~msvs:true opam_master_hash opam_branches
   in
   (* 2022-10-12: Docker Engine 20.10.18 on Windows fails copying
      C:\cygwin64, so we cannot build Cygwin in a separate image. *)
   let ocaml_for_windows =
-    let ofw_packages, ofw_setup = Windows.Cygwin.install_ocaml_for_windows () in
-    vs_build_tools
-    @@ Windows.Cygwin.install_cygwin ~msvs_tools:true
-         ~extra:(packages @ ofw_packages) ()
-    @@ ofw_setup
+    let packages, setup = Windows.Cygwin.install_ocaml_for_windows () in
+    Windows.Cygwin.install packages @@ setup
   in
   parser_directive (`Escape '`')
   @@ comment "Autogenerated by OCaml-Dockerfile scripts"
-  @@ winget_image @@ opams_image
-  @@ header ?win10_revision ?arch distro
+  @@ winget_image @@ cygwin_msvc_image @@ opams_image
+  @@ Dockerfile.from "cygwin-msvc"
   @@ label (("distro_style", "windows") :: labels)
   @@ user "ContainerAdministrator"
-  @@ Windows.sanitize_reg_path ()
   @@ ocaml_for_windows @@ winget_setup
   @@ copy_opams_windows opam_branches
   @@ Windows.Cygwin.setup () @@ Windows.Cygwin.Git.init ()