release database connections after thread termination of the "kill after timeout thread"

lib/docker_client.rb

@@ -174,6 +174,7 @@ class DockerClient
     as it is impossible to determine whether further input is requested.
     """
     @thread = Thread.new do
+      begin
         timeout = @execution_environment.permitted_execution_time.to_i # seconds
         sleep(timeout)
         if container.status != :returned
@@ -184,6 +185,10 @@ class DockerClient
           end
           kill_container(container)
         end
+      ensure
+        #guarantee that the thread is releasing the DB connection after it is done
+        ActiveRecord::Base.connectionpool.releaseconnection
+      end
     end
   end