Plugin/slack does't work

Hi, when I use plugin/slack to notice in the Mattermost, the pipeline seems show something
wrong which like below:
2019/03/27 11:05:26 Post dial tcp connect: no route to host
But when I run the docker image, plugin/slack, it works. Only in the pipeline, it doesn’t work.

Sorry to my poor english, wish you can understand what i said.