Use ping option only for DCP WebSocket

* The Faye::WebSocket library will "buffer" some output of the connection and emit the `on :message` events in the order of the messages. However, when a ping is sent while the connection has already been closed, it will emit the `on :close` event immediately and drop all other messages (in that "buffer"). This is problematic for very short running executions that generate a long output (as this will be cut off without a proper exit message sent by Poseidon).

lib/runner/strategy/docker_container_pool.rb

@@ -129,7 +129,11 @@ class Runner::Strategy::DockerContainerPool < Runner::Strategy
   end
 
   def self.websocket_header
-    {}
+    # The `ping` value is measured in seconds and specifies how often a Ping frame should be sent.
+    # Internally, Faye::WebSocket uses EventMachine and the `ping` value is used to wake the EventMachine thread
+    {
+      ping: 0.1,
+    }
   end
 
   private