nixos/nixos-containers: add restartIfChanged option

This commit makes auto-restarting declarative containers optional.

This is useful when you don't want changes to your config to automatically
restart the container and thus stop any applications running inside it.

nixos/modules/virtualisation/nixos-containers.nix

@@ -649,6 +649,15 @@ in
               '';
             };
 
+            restartIfChanged = mkOption {
+              type = types.bool;
+              default = true;
+              description = lib.mdDoc ''
+                Whether the container should be restarted during a NixOS
+                configuration switch if its definition has changed.
+              '';
+            };
+
             timeoutStartSec = mkOption {
               type = types.str;
               default = "1min";
@@ -826,7 +835,7 @@ in
                 containerConfig.path
                 config.environment.etc."${configurationDirectoryName}/${name}.conf".source
               ];
-              restartIfChanged = true;
+              restartIfChanged = containerConfig.restartIfChanged;
             }
           )
       )) config.containers)