Replace /bin/sh, /usr/bin/env mounts by symlinks

This makes the arion-base image compatible with hosts other than
NixOS.

arion.nix

@@ -11,6 +11,7 @@ stdenv.mkDerivation {
       mkdir -p $out/bin $out/share/arion
       cp -a nix $out/share/arion/
       cp -a arion-image $out/share/arion/
+      tar -czf $out/share/arion/arion-image/tarball.tar.gz -C arion-image/tarball .
       substitute arion $out/bin/arion \
         --subst-var-by path ${lib.makeBinPath [jq coreutils docker_compose]} \
         --subst-var-by nix_dir $out/share/arion/nix \

src/arion-image/Dockerfile

@@ -1,2 +1,3 @@
 FROM scratch
 COPY passwd /etc/passwd
+ADD tarball.tar.gz /

src/arion-image/tarball/bin/sh

@@ -0,0 +1 @@
+/run/system/bin/sh

src/arion-image/tarball/usr/bin/env

@@ -0,0 +1 @@
+/run/system/usr/bin/env

src/nix/service-host-store.nix

@@ -4,7 +4,7 @@
    when the service.useHostStore option is set to true.
 
  */
-{ lib, config, ... }:
+{ lib, config, pkgs, ... }:
 let
   inherit (lib) mkOption types mkIf;
 in
@@ -21,8 +21,7 @@ in
     service.build.context = "${../arion-image}";
     service.volumes = [
       "/nix/store:/nix/store"
-      "/bin/sh:/bin/sh"
-      "/usr/bin/env:/usr/bin/env"
+      "${pkgs.buildEnv { name = "container-system-env"; paths = [ pkgs.bashInteractive pkgs.coreutils ]; }}:/run/system"
     ];
   };
 }