I have this exact issue.
gitlab & mattermost running together (in my case in the same docker container).
As per gitlab documentation, it is enough to put the certificate under:
/etc/gitlab/trusted-certs/my_full_chain_ca.crt
After reconfigure, I do find it linked under: /opt/gitlab/embedded/ssl/certs/ but mattermost is still complaining with:
[2016/10/22 22:34:58 IDT] [EROR] /signup/gitlab/complete:AuthorizeOAuthUser code=500 rid=dddddddddddduid= ip=10.x.x.x Token request failed [details: Post https://gitlab.aaaaa.com/oauth/token: x509: certificate signed by unknown authority]
I checked, and my_full_chain_ca.crt is not linked automatically under: /etc/ssl/certs. Should it be there? If so, shouldn't reconfigure take care of this as well?