Mattermost SSO doesn't pick-up custom CA certs from /etc/gitlab/trusted-certs during OAUTH token POST

Docker image: 10.3.2-ee.0

I have successfully set-up HTTPS using custom certificates on my GitLab UI, GitLab Docker and GitLab Mattermost URL's. All these URL are working fine with my custom CA certificate using the following folders:

# ls -la /etc/gitlab/trusted-certs
total 4
drwxr-xr-x. 3 root root  100 Dec 29 16:32 .
drwxrwsr-x. 6 root 2004 4096 Dec 29 15:09 ..
drwxr-xr-x. 2 root root   60 Dec 29 16:32 ..129812_29_12_17_32_05.675294905
lrwxrwxrwx. 1 root root   33 Dec 29 16:32 ..data -> ..129812_29_12_17_32_05.675294905
lrwxrwxrwx. 1 root root   29 Dec 29 16:06 DigicertBundle.crt -> ..data/DigicertBundle.crt


# ls -la /etc/gitlab/ssl
total 4
drwxrwxrwt. 3 root root  200 Dec 29 16:06 .
drwxrwsr-x. 6 root 2004 4096 Dec 29 15:09 ..
drwxr-xr-x. 2 root root  160 Dec 29 16:06 ..129812_29_12_17_06_40.771912950
lrwxrwxrwx. 1 root root   33 Dec 29 16:06 ..data -> ..129812_29_12_17_06_40.771912950
lrwxrwxrwx. 1 root root   53 Dec 29 16:06 docker-gitlab.mydomain.com.crt -> ..data/docker-gitlab.mydomain.com.crt
lrwxrwxrwx. 1 root root   53 Dec 29 16:06 docker-gitlab.mydomain.com.key -> ..data/docker-gitlab.mydomain.com.key
lrwxrwxrwx. 1 root root   46 Dec 29 16:06 gitlab.mydomain.com.crt -> ..data/gitlab.mydomain.com.crt
lrwxrwxrwx. 1 root root   46 Dec 29 16:06 gitlab.mydomain.com.key -> ..data/gitlab.mydomain.com.key
lrwxrwxrwx. 1 root root   50 Dec 29 16:06 mattermost.mydomain.com.crt -> ..data/mattermost.mydomain.com.crt
lrwxrwxrwx. 1 root root   50 Dec 29 16:06 mattermost.mydomain.com.key -> ..data/mattermost.mydomain.com.key

# tail -n 5 /var/log/gitlab/mattermost/current
2017-12-29_16:15:08.27549 segment 2017/12/29 16:15:08 error sending request: Post https://api.segment.io/v1/batch: dial tcp 54.68.145.115:443: i/o timeout
2017-12-29_16:26:18.42395 [2017/12/29 16:26:18 UTC] [EROR] AuthorizeOAuthUser: Token request failed, Post https://gitlab.mydomain.com/oauth/token: x509: certificate signed by unknown authority
2017-12-29_16:26:58.35365 [2017/12/29 16:26:58 UTC] [EROR] AuthorizeOAuthUser: Token request failed, Post https://gitlab.mydomain.com/oauth/token: x509: certificate signed by unknown authority
2017-12-29_16:27:55.60548 [2017/12/29 16:27:55 UTC] [EROR] AuthorizeOAuthUser: Token request failed, Post https://gitlab.mydomain.com/oauth/token: x509: certificate signed by unknown authority
2017-12-29_16:31:43.77154 [2017/12/29 16:31:43 UTC] [EROR] AuthorizeOAuthUser: Token request failed, Post https://gitlab.mydomain.com/oauth/token: x509: certificate signed by unknown authority

As one can see in the above traces, the Mattermost process is not picking up this valid CA certificate chain, during it's OAUTH SSO cycle at token POST'ing. I am pretty sure the CA bundle is correct, otherwise the other working HTTPS endpoints would not work as well.