Add Docker MTU configuration support for networks with non-standard MTU (#6079)

* Initial plan

* Implement Docker MTU support - core functionality

Co-authored-by: agners <34061+agners@users.noreply.github.com>

* Add comprehensive MTU tests and documentation

Co-authored-by: agners <34061+agners@users.noreply.github.com>

* Fix final linting issue in test file

Co-authored-by: agners <34061+agners@users.noreply.github.com>

* Apply suggestions from code review

* Implement reboot_required flag pattern and fix MyPy typing issue

Co-authored-by: agners <34061+agners@users.noreply.github.com>

* Update supervisor/api/docker.py

* Update supervisor/docker/manager.py

Co-authored-by: Mike Degatano <michael.degatano@gmail.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: agners <34061+agners@users.noreply.github.com>
Co-authored-by: Stefan Agner <stefan@agner.ch>
Co-authored-by: Mike Degatano <michael.degatano@gmail.com>

tests/api/test_docker.py

@@ -32,6 +32,52 @@ async def test_api_network_enable_ipv6(coresys: CoreSys, api_client: TestClient)
     assert body["data"]["enable_ipv6"] is True
 
 
+async def test_api_network_mtu(coresys: CoreSys, api_client: TestClient):
+    """Test setting docker network MTU."""
+    assert coresys.docker.config.mtu is None
+
+    resp = await api_client.post("/docker/options", json={"mtu": 1450})
+    assert resp.status == 200
+
+    assert coresys.docker.config.mtu == 1450
+
+    resp = await api_client.get("/docker/info")
+    assert resp.status == 200
+    body = await resp.json()
+    assert body["data"]["mtu"] == 1450
+
+    # Test setting MTU to None
+    resp = await api_client.post("/docker/options", json={"mtu": None})
+    assert resp.status == 200
+
+    assert coresys.docker.config.mtu is None
+
+    resp = await api_client.get("/docker/info")
+    assert resp.status == 200
+    body = await resp.json()
+    assert body["data"]["mtu"] is None
+
+
+async def test_api_network_combined_options(coresys: CoreSys, api_client: TestClient):
+    """Test setting both IPv6 and MTU together."""
+    assert coresys.docker.config.enable_ipv6 is None
+    assert coresys.docker.config.mtu is None
+
+    resp = await api_client.post(
+        "/docker/options", json={"enable_ipv6": True, "mtu": 1400}
+    )
+    assert resp.status == 200
+
+    assert coresys.docker.config.enable_ipv6 is True
+    assert coresys.docker.config.mtu == 1400
+
+    resp = await api_client.get("/docker/info")
+    assert resp.status == 200
+    body = await resp.json()
+    assert body["data"]["enable_ipv6"] is True
+    assert body["data"]["mtu"] == 1400
+
+
 async def test_registry_not_found(api_client: TestClient):
     """Test registry not found error."""
     resp = await api_client.delete("/docker/registries/bad")