Load docker api only for Linux

Fix #525

gns3server/handlers/__init__.py

@@ -24,7 +24,6 @@ from gns3server.handlers.api.dynamips_device_handler import DynamipsDeviceHandle
 from gns3server.handlers.api.dynamips_vm_handler import DynamipsVMHandler
 from gns3server.handlers.api.qemu_handler import QEMUHandler
 from gns3server.handlers.api.virtualbox_handler import VirtualBoxHandler
-from gns3server.handlers.api.docker_handler import DockerHandler
 from gns3server.handlers.api.vpcs_handler import VPCSHandler
 from gns3server.handlers.api.vmware_handler import VMwareHandler
 from gns3server.handlers.api.config_handler import ConfigHandler
@@ -34,6 +33,8 @@ from gns3server.handlers.upload_handler import UploadHandler
 from gns3server.handlers.index_handler import IndexHandler
 
 if sys.platform.startswith("linux") or hasattr(sys, "_called_from_test") or os.environ.get("PYTEST_BUILD_DOCUMENTATION") == "1":
+    from gns3server.handlers.api.docker_handler import DockerHandler
+
     # IOU runs only on Linux but testsuite work on UNIX platform
     if not sys.platform.startswith("win"):
         from gns3server.handlers.api.iou_handler import IOUHandler