docker: don't recreate containers on empty-string property PUTs

Web clients serialize empty form fields as "" while unset values are
stored as None on the node. The bare != diff in the update handler then
sees a phantom change on every full PUT and recreates the container for
nothing -- even when the user only changed a controller-only field such
as netmiko_device_type.

Normalize at the schema boundary ("" -> None for start_command,
environment and extra_hosts; "" -> "/" for console_http_path), make
the setters apply the same canonicalization, and create nodes through
the setters instead of bypassing them in __init__ so both paths store
identical values.

gns3server/compute/docker/docker_vm.py

@@ -129,8 +129,10 @@ class DockerVM(BaseNode):
         if ":" not in image:
             image = f"{image}:latest"
         self._image = image
-        self._start_command = start_command
-        self._environment = environment
+        # assign through the property setters so creation and updates apply
+        # the same value normalization (e.g. "" -> None)
+        self.start_command = start_command
+        self.environment = environment
         self._cid = None
         self._ethernet_adapters = []
         self._temporary_directory = None
@@ -138,10 +140,10 @@ class DockerVM(BaseNode):
         self._vnc_process = None
         self._vncconfig_process = None
         self._console_resolution = console_resolution
-        self._console_http_path = console_http_path
+        self.console_http_path = console_http_path
         self._console_http_port = console_http_port
         self._console_websocket = None
-        self._extra_hosts = extra_hosts
+        self.extra_hosts = extra_hosts
         self._extra_volumes = extra_volumes or []
         self._extra_configs = extra_configs or []
         self._memory = memory
@@ -288,7 +290,9 @@ class DockerVM(BaseNode):
 
     @console_http_path.setter
     def console_http_path(self, path):
-        self._console_http_path = path
+        # the canonical "no path" value is "/" so that "", None and "/"
+        # all compare equal in the update diff
+        self._console_http_path = path or "/"
 
     @property
     def console_http_port(self):
@@ -304,7 +308,8 @@ class DockerVM(BaseNode):
 
     @environment.setter
     def environment(self, command):
-        self._environment = command
+        # "" and None are the same "no environment variables" value
+        self._environment = command or None
 
     @property
     def extra_hosts(self):
@@ -312,7 +317,8 @@ class DockerVM(BaseNode):
 
     @extra_hosts.setter
     def extra_hosts(self, extra_hosts):
-        self._extra_hosts = extra_hosts
+        # "" and None are the same "no extra hosts" value
+        self._extra_hosts = extra_hosts or None
 
     @property
     def extra_volumes(self):

gns3server/schemas/compute/docker_nodes.py

@@ -14,7 +14,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, field_validator
 from typing import Optional, List
 from uuid import UUID
 
@@ -26,6 +26,21 @@ class DockerBase(BaseModel):
     Common Docker node properties.
     """
 
+    @field_validator("start_command", "environment", "extra_hosts", mode="before")
+    @classmethod
+    def _empty_string_to_none(cls, value):
+        # Web clients serialize empty form fields as "" while unset values are
+        # stored as None on the node: normalize before the update diff runs,
+        # otherwise every full PUT would see a phantom change and recreate
+        # the container for nothing.
+        return value or None
+
+    @field_validator("console_http_path", mode="before")
+    @classmethod
+    def _empty_string_to_root_path(cls, value):
+        # the canonical "no path" value is "/" (the creation default)
+        return value or "/"
+
     name: str
     image: str = Field(..., description="Docker image name")
     node_id: Optional[UUID] = None

tests/api/routes/compute/test_docker_nodes.py

@@ -306,6 +306,45 @@ class TestDockerNodesRoutes:
         assert response.json()["environment"] == "GNS3=1\nGNS4=0"
         assert response.json()["extra_hosts"] == "test:127.0.0.1"
 
+    async def test_docker_update_empty_strings_do_not_recreate_container(
+            self,
+            app: FastAPI,
+            compute_client: AsyncClient,
+            compute_project: Project
+    ) -> None:
+        """
+        Web clients serialize empty form fields as "" while unset values are
+        stored as None on the node: a full PUT must not see a phantom change
+        and recreate the container for nothing.
+        """
+
+        params = {"name": "DOCKER-EMPTY", "image": "nginx", "environment": ""}
+        with asyncio_patch("gns3server.compute.docker.Docker.list_images", return_value=[{"image": "nginx"}]):
+            with asyncio_patch("gns3server.compute.docker.Docker.query", return_value={"Id": "8bd8153ea8f5"}):
+                response = await compute_client.post(
+                    app.url_path_for("compute:create_docker_node", project_id=compute_project.id), json=params
+                )
+        assert response.status_code == status.HTTP_201_CREATED
+        assert response.json()["environment"] is None  # "" normalized at creation
+        assert response.json()["console_http_path"] == "/"
+        node_id = response.json()["node_id"]
+
+        with asyncio_patch("gns3server.compute.docker.docker_vm.DockerVM.update") as mock:
+            response = await compute_client.put(
+                app.url_path_for("compute:update_docker_node", project_id=compute_project.id, node_id=node_id),
+                json={
+                    "name": "DOCKER-EMPTY",
+                    "start_command": "",
+                    "environment": "",
+                    "extra_hosts": "",
+                    "console_http_path": "",
+                },
+            )
+        assert response.status_code == 200
+        assert not mock.called  # no real change: the container must not be recreated
+        assert response.json()["start_command"] is None
+        assert response.json()["console_http_path"] == "/"
+
 
     async def test_docker_start_capture(self, app: FastAPI, compute_client: AsyncClient, vm: dict) -> None: