fakeNss: move to toplevel

Make this reachable from pkgs.fakeNss. This is useful outside docker
contexts, too.

https://github.com/NixOS/nixpkgs/pull/164943#discussion_r833220769

pkgs/build-support/docker/default.nix

@@ -6,6 +6,7 @@
 , coreutils
 , e2fsprogs
 , fakechroot
+, fakeNss
 , fakeroot
 , findutils
 , go
@@ -747,25 +748,7 @@ rec {
   # Useful when packaging binaries that insist on using nss to look up
   # username/groups (like nginx).
   # /bin/sh is fine to not exist, and provided by another shim.
-  fakeNss = symlinkJoin {
-    name = "fake-nss";
-    paths = [
-      (writeTextDir "etc/passwd" ''
-        root:x:0:0:root user:/var/empty:/bin/sh
-        nobody:x:65534:65534:nobody:/var/empty:/bin/sh
-      '')
-      (writeTextDir "etc/group" ''
-        root:x:0:
-        nobody:x:65534:
-      '')
-      (writeTextDir "etc/nsswitch.conf" ''
-        hosts: files dns
-      '')
-      (runCommand "var-empty" { } ''
-        mkdir -p $out/var/empty
-      '')
-    ];
-  };
+  inherit fakeNss; # alias
 
   # This provides a /usr/bin/env, for shell scripts using the
   # "#!/usr/bin/env executable" shebang.

pkgs/build-support/fake-nss/default.nix

@@ -0,0 +1,24 @@
+# Provide a /etc/passwd and /etc/group that contain root and nobody.
+# Useful when packaging binaries that insist on using nss to look up
+# username/groups (like nginx).
+# /bin/sh is fine to not exist, and provided by another shim.
+{ symlinkJoin, writeTextDir, runCommand }:
+symlinkJoin {
+  name = "fake-nss";
+  paths = [
+    (writeTextDir "etc/passwd" ''
+      root:x:0:0:root user:/var/empty:/bin/sh
+      nobody:x:65534:65534:nobody:/var/empty:/bin/sh
+    '')
+    (writeTextDir "etc/group" ''
+      root:x:0:
+      nobody:x:65534:
+    '')
+    (writeTextDir "etc/nsswitch.conf" ''
+      hosts: files dns
+    '')
+    (runCommand "var-empty" { } ''
+      mkdir -p $out/var/empty
+    '')
+  ];
+}

pkgs/top-level/all-packages.nix

@@ -436,6 +436,9 @@ with pkgs;
   dockerTools = callPackage ../build-support/docker {
     writePython3 = buildPackages.writers.writePython3;
   };
+
+  fakeNss = callPackage ../build-support/fake-nss { };
+
   tarsum = callPackage ../build-support/docker/tarsum.nix { };
 
   snapTools = callPackage ../build-support/snap { };