dockerTools: Allow omitting all store paths

Adds includeStorePaths, allowing the omission of the store paths.
You generally want to leave it on, but tooling may disable this
to insert the store paths more efficiently via other means, such
as bind mounting the host store.

nixos/tests/docker-tools.nix

@@ -20,6 +20,20 @@ import ./make-test-python.nix ({ pkgs, ... }: {
 
     docker.wait_for_unit("sockets.target")
 
+    with subtest("includeStorePath"):
+        with subtest("assumption"):
+            docker.succeed("${examples.helloOnRoot} | docker load")
+            docker.succeed("set -euo pipefail; docker run --rm hello | grep -i hello")
+            docker.succeed("docker image rm hello:latest")
+        with subtest("includeStorePath = false; breaks example"):
+            docker.succeed("${examples.helloOnRootNoStore} | docker load")
+            docker.fail("set -euo pipefail; docker run --rm hello | grep -i hello")
+            docker.succeed("docker image rm hello:latest")
+        with subtest("includeStorePath = false; works with mounted store"):
+            docker.succeed("${examples.helloOnRootNoStore} | docker load")
+            docker.succeed("set -euo pipefail; docker run --rm --volume ${builtins.storeDir}:${builtins.storeDir}:ro hello | grep -i hello")
+            docker.succeed("docker image rm hello:latest")
+
     with subtest("Ensure Docker images use a stable date by default"):
         docker.succeed(
             "docker load --input='${examples.bash}'"

pkgs/build-support/docker/default.nix

@@ -37,6 +37,10 @@
 
 let
 
+  inherit (lib)
+    optionals
+    ;
+
   mkDbExtraCommand = contents: let
     contentsList = if builtins.isList contents then contents else [ contents ];
   in ''
@@ -787,6 +791,10 @@ rec {
     # We pick 100 to ensure there is plenty of room for extension. I
     # believe the actual maximum is 128.
     maxLayers ? 100,
+    # Whether to include store paths in the image. You generally want to leave
+    # this on, but tooling may disable this to insert the store paths more
+    # efficiently via other means, such as bind mounting the host store.
+    includeStorePaths ? true,
   }:
     assert
       (lib.assertMsg (maxLayers > 1)
@@ -834,7 +842,9 @@ rec {
         '';
       };
 
-      closureRoots = [ baseJson ] ++ contentsList;
+      closureRoots = optionals includeStorePaths /* normally true */ (
+        [ baseJson ] ++ contentsList
+      );
       overallClosure = writeText "closure" (lib.concatStringsSep " " closureRoots);
 
       # These derivations are only created as implementation details of docker-tools,

pkgs/build-support/docker/examples.nix

@@ -516,4 +516,29 @@ rec {
     bash
     layeredImageWithFakeRootCommands
   ];
+
+  helloOnRoot = pkgs.dockerTools.streamLayeredImage {
+    name = "hello";
+    tag = "latest";
+    contents = [
+      (pkgs.buildEnv {
+        name = "hello-root";
+        paths = [ pkgs.hello ];
+      })
+    ];
+    config.Cmd = [ "hello" ];
+  };
+
+  helloOnRootNoStore = pkgs.dockerTools.streamLayeredImage {
+    name = "hello";
+    tag = "latest";
+    contents = [
+      (pkgs.buildEnv {
+        name = "hello-root";
+        paths = [ pkgs.hello ];
+      })
+    ];
+    config.Cmd = [ "hello" ];
+    includeStorePaths = false;
+  };
 }