docker: persist extra_configs in the template DB table

The extra_configs schema field needs a column on the docker_templates table
to actually round-trip through the controller DB (the schema alone is
accepted but dropped by the SQLAlchemy model mapping). Add the JSON column
and an Alembic migration so existing databases get it on upgrade.

gns3server/db/models/templates.py

@@ -78,6 +78,7 @@ class DockerTemplate(Template):
     console_resolution = Column(String)
     extra_hosts = Column(String)
     extra_volumes = Column(JSON)
+    extra_configs = Column(JSON)
     memory = Column(Integer)
     cpus = Column(Float)
     custom_adapters = Column(JSON)