Move uid and customNixRootPath into config.host module

Also make the type of host.uid consistently int.

src/nix/eval-composition.nix

@@ -1,4 +1,4 @@
-{ modules ? [], uid ? 0, pkgs, customNixRootPath ? "", }:
+{ modules ? [], uid ? "0", pkgs, hostNixStorePrefix ? "", }:
 
 let _pkgs = pkgs;
 in
@@ -19,14 +19,15 @@ let
   builtinModules = [
     argsModule
     ./modules/composition/docker-compose.nix
+    ./modules/composition/host-environment.nix
   ];
 
   argsModule = {
     _file = ./eval-composition.nix;
     key = ./eval-composition.nix;
     config._module.args.pkgs = lib.mkIf (pkgs != null) (lib.mkForce pkgs);
-    config._module.args.uid = uid;
-    config._module.args.customNixRootPath = customNixRootPath;
+    config.host.nixStorePrefix = hostNixStorePrefix;
+    config.host.uid = lib.toInt uid;
   };
 
 in

src/nix/modules/composition/docker-compose.nix

@@ -9,7 +9,7 @@
     - docker-compose.services
 
  */
-{ pkgs, uid, lib, config, customNixRootPath, ... }:
+{ pkgs, lib, config, ... }:
 
 let
   evalService = name: modules:
@@ -23,14 +23,14 @@ let
         argsModule
         ../service/docker-compose-service.nix
         ../service/host-store.nix
+        ../service/host.nix
       ];
 
       argsModule = {
         _file = ./docker-compose.nix;
         key = ./docker-compose.nix;
         config._module.args.pkgs = lib.mkForce pkgs;
-        config._module.args.uid = uid;
-        config._module.args.customNixRootPath = customNixRootPath;
+        config.host = config.host;
       };
 
     in

src/nix/modules/composition/host-environment.nix

@@ -0,0 +1,32 @@
+{ lib, ... }:
+
+{
+  options = {
+
+    host.uid = lib.mkOption {
+      type = lib.types.int;
+      description = ''
+        The numeric user id (UID) of the user running arion.
+
+        Assuming this user id is helpful when dealing with the user's
+        files, mounted into the container as a volume.
+      '';
+    };
+
+    host.nixStorePrefix = lib.mkOption {
+      type = lib.types.string;
+      default = "";
+      example = "/mnt/foo";
+      description = ''
+        Prefixes store paths on the host, allowing the Nix store to be
+        stored at an alternate location without altering the format of
+        store paths.
+
+        For example: instead of mounting the host's /nix/store as the
+        container's /nix/store, this will mount /mnt/foo/nix/store
+        as the container's /nix/store.
+      '';
+    };
+
+  };
+}

src/nix/modules/service/host-store.nix

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

src/nix/modules/service/host.nix

@@ -0,0 +1,11 @@
+{ lib, ... }:
+{
+  options = {
+    host = lib.mkOption {
+      type = lib.types.attrs;
+      description = ''
+        The composition-level host option values.
+      '';
+    };
+  };
+}