Mattermost log error 404 with integration hooks

I had the same issue. My Setup is this:

Hostsystem - Nginx Reverse Proxy for Mattermost and Gitlab)
- Gitlab Docker Container
- Mattermost Docker Container

I first installed the mattermost gitlab integration on the Hostsystem, but that didn't work.

I then installed the mattermost gitlab integration in the Gitlab Docker instance and configured the Gitlab projects webhook url:

http://localhost:5000/new_event

and disabled the SSL Verification. That worked for me but i don't like it.

You can probably set it up on the hostsystem and use nginx reverse proxy for ssl.