specify API version in url used to open the websocket. With version 1.28 and up, docker uses binary frame format for websockets which does not seem to work with FAYE websocket. For API version 1.27 and below, text frame format is used.

+ short fix improvement for empty responses

lib/docker_client.rb

@@ -72,7 +72,7 @@ class DockerClient
     # Headers are required by Docker
     headers = {'Origin' => 'http://localhost'}
 
-    socket_url = DockerClient.config['ws_host'] + '/containers/' + @container.id + '/attach/ws?' + query_params
+    socket_url = DockerClient.config['ws_host'] + '/v1.27/containers/' + @container.id + '/attach/ws?' + query_params
     socket = Faye::WebSocket::Client.new(socket_url, [], :headers => headers)
 
     Rails.logger.debug "Opening Websocket on URL " + socket_url
@@ -399,6 +399,9 @@ class DockerClient
       output = container.exec(['bash', '-c', command])
       Rails.logger.debug "output from container.exec"
       Rails.logger.debug output
+      if(output == nil)
+        kill_container(container)
+      end
       result = {status: output[2] == 0 ? :ok : :failed, stdout: output[0].join.force_encoding('utf-8'), stderr: output[1].join.force_encoding('utf-8')}
     end
     # if we use pooling and recylce the containers, put it back. otherwise, destroy it.