Cache Apt package downloads with BuildKit cache mounts

CHANGES.md

@@ -1,3 +1,9 @@
+unreleased
+----------
+
+- Cache packages downloads for Apt-based distributions using BuildKit
+  cache mounts. (@MisterDA, #???)
+
 v8.2.3 2024-10-30 Rochester
 ---------------------------
 

src-opam/linux.ml

@@ -76,14 +76,27 @@ end
 
 (** Debian rules *)
 module Apt = struct
+  (* https://docs.docker.com/reference/dockerfile/#example-cache-apt-packages *)
+  let mounts =
+    [
+      mount_cache ~target:"/var/cache/apt" ~sharing:`Locked ();
+      mount_cache ~target:"/var/lib/apt" ~sharing:`Locked ();
+    ]
+
   let update =
-    run "apt-get -y update"
-    @@ run "DEBIAN_FRONTEND=noninteractive apt-get -y upgrade"
+    run
+      {|rm -f /etc/apt/apt.conf.d/docker-clean; echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache|}
+    @@ run ~mounts
+         "apt update && DEBIAN_FRONTEND=noninteractive apt-get -y upgrade"
 
   let install fmt =
     ksprintf
       (fun s ->
-        update @@ run "DEBIAN_FRONTEND=noninteractive apt-get -y install %s" s)
+        update
+        @@ run ~mounts
+             "DEBIAN_FRONTEND=noninteractive apt-get --no-install-recommends \
+              install -y %s"
+             s)
       fmt
 
   let dev_packages ?extra () =

src-opam/linux.mli

@@ -59,7 +59,7 @@ end
 (** Rules for Apt-based distributions *)
 module Apt : sig
   val update : t
-  (** [update] will run [apt-get update && apt-get upgrade] non-interactively. *)
+  (** [update] will run [apt-get update && apt-get upgrade] non-interactively. Requires [syntax=docker/dockerfile:1]. *)
 
   val install : ('a, unit, string, t) format4 -> 'a
   (** [install fmt] will [apt-get update && apt-get install] the packages specified by the [fmt] format string. *)