Hi Eunjae,
I took another look and the site you linked is using our Trusty
build image, which is no longer getting bugfixes. We strongly recommend that customers select the Xenial
image for builds. You can read more about the differences here: [Support Guide] [Deprecated] What are the differences between the Trusty and Xenial build images?
Would upgrading to the Xenial
image be possible for your team?