[8.17-RC1] WSS for mattermost has issue with cross-origin

This is a bug report for docker omnibus gitlab-ce image for 8.17.0-rc1.ce.0

I use a reverse proxy to serve both gitlab and mattermost. On this version, mattermost is not working anymore because of some cross-origin issues, whereas it was working perfectly well in previous version.

A rollback to 8.16.4 did the job, mattermost works again perfectly well.

Issue specifically is with the wss://disc.tutu.com:443/api/v3/users/websocket that sens a 500 error.

In mattermost logs, I see that the origin is not allowed.

[2017/02/11 15:39:49 UTC] [EROR] websocket connect err: websocket: origin not allowed
[2017/02/11 15:39:49 UTC] [EROR] /api/v3/users/websocket:connect code=500 rid=x6foapujtprsufxri8fyxucjie uid=nnqmxgkn5tbxjg98yhq5y664ze ip=5.51.59.153, 192.168.0.4 Failed to upgrade websocket connection [details: ]