nixos/nixos-containers: add flake option

While the nixos-container command allows for the creation of containers pointing to a flake, the declarative module doesn't have this option.
Adds the flake option for nixos-container declarative approach. Creates the /etc/nixos-container .conf file similar to how the command preforms it.

nixos/modules/virtualisation/nixos-containers.nix

@@ -844,6 +844,16 @@ in
                 '';
               };
 
+              flake = lib.mkOption {
+                type = lib.types.nullOr lib.types.str;
+                default = null;
+                example = "github:NixOS/nixpkgs/master";
+                description = ''
+                  The Flake URI of the NixOS configuration to use for the container.
+                  Replaces the option {option}`containers.<name>.path`.
+                '';
+              };
+
               # Removed option. See `checkAssertion` below for the accompanying error message.
               pkgs = mkOption { visible = false; };
             } // networkOptions;
@@ -867,13 +877,18 @@ in
                       - containers.${name}.config.nixpkgs.pkgs
                         This only sets the `pkgs` argument used inside the container modules.
                     ''
+                  else if options.config.isDefined && (options.flake.value != null) then
+                    throw ''
+                      The options 'containers.${name}.path' and 'containers.${name}.flake' cannot both be set.
+                    ''
                   else
                     null;
               in
               {
-                path =
-                  builtins.seq checkAssertion mkIf options.config.isDefined
-                    config.config.system.build.toplevel;
+                path = builtins.seq checkAssertion mkMerge [
+                  (mkIf options.config.isDefined config.config.system.build.toplevel)
+                  (mkIf (config.flake != null) "/nix/var/nix/profiles/per-container/${name}")
+                ];
               };
           }
         )
@@ -1044,7 +1059,12 @@ in
             name: cfg:
             nameValuePair "${configurationDirectoryName}/${name}.conf" {
               text = ''
-                SYSTEM_PATH=${cfg.path}
+                ${optionalString (cfg.flake == null) ''
+                  SYSTEM_PATH=${cfg.path}
+                ''}
+                ${optionalString (cfg.flake != null) ''
+                  FLAKE=${cfg.flake}
+                ''}
                 ${optionalString cfg.privateNetwork ''
                   PRIVATE_NETWORK=1
                   ${optionalString (cfg.hostBridge != null) ''