appliance: accept docker_exec console type in Docker appliances

The Docker appliance Pydantic model (DockerConsoleType) rejected
console_type='docker_exec', so an appliance file using the vendor NOS
docker_exec console could not be loaded (validation error at import).
Add docker_exec to the enum — it is already a valid ConsoleType
(schemas/common.py) and is handled by VendorDockerVM.

gns3server/schemas/controller/appliances.py

@@ -285,6 +285,7 @@ class DockerConsoleType(str, Enum):
     http = 'http'
     https = 'https'
     none = 'none'
+    docker_exec = 'docker_exec'
 
 
 class ChecksumType(str, Enum):