implemented pooling for Docker containers

db/migrate/20150204080832_add_pool_size_to_execution_environments.rb

@@ -0,0 +1,11 @@
+class AddPoolSizeToExecutionEnvironments < ActiveRecord::Migration
+  def change
+    add_column :execution_environments, :pool_size, :integer
+
+    reversible do |direction|
+      direction.up do
+        ExecutionEnvironment.update_all(pool_size: 0)
+      end
+    end
+  end
+end