Document external Mattermost self-signed certificate handling

Pavan Kumar Sadashivrequested to merge
gitlab-community/gitlab-org/omnibus-gitlab:hrlpavan-doc-mattermost-docker-ssl into master

What does this MR do and why?

Documents how to configure self-signed SSL certificates when integrating an external Mattermost instance with GitLab to resolve x509: certificate signed by unknown authority errors:

  • Mattermost connecting to GitLab (such as during GitLab OAuth /oauth/token token exchange): adding the GitLab self-signed or root CA certificate to the system CA trust store on the Mattermost container or host.
  • GitLab connecting to Mattermost (such as notifications or slash commands): adding the Mattermost certificate to /etc/gitlab/trusted-certs/ on the GitLab instance.

Closes #841 (closed)

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Edited by Pavan Kumar Sadashiv

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