Webhook SSL verification failed after adding custom CA certificate

Summary

After adding the self-signed CA certificate into GitLab according to README.html#using-self-signed-certificate-or-custom-certificate-authorities my webhook still fails with SSL verification enabled:

Hook execution failed: SSL_connect returned=1 errno=0 state=error: certificate verify failed

The symlink inside /opt/gitlab/embedded/ssl/certs/was created successfully pointing to my custom certificate. I've also tried to add the self-signed certificate of my webhook endpoint itself without success - still the same error.

Self-signed certificates were also enabled inside /etc/gitlab/gitlab.rb:

gitlab_shell['http_settings'] = { user: 'username', password: 'password', ca_file: '/etc/ssl/cert.pem', ca_path: '/etc/pki/tls/certs', self_signed_cert: true}

SystemEnvironment

• Current GitLab CE Version: 8.9.0 (upgrading from 8.8.5)
• Running the official Docker Image gitlab/gitlab-ce:8.9.0-ce.0

GitLab Environment Info

System information
System: 
Current User:   git
Using RVM:      no
Ruby Version:   2.1.8p440
Gem Version:    2.5.1
Bundler Version:1.10.6
Rake Version:   10.5.0
Sidekiq Version:4.1.2

GitLab information
Version:        8.9.0
Revision:       0d457d8
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
URL:            https://git.example.org
HTTP Clone URL: https://git.example.org/some-group/some-project.git
SSH Clone URL:  git@git.example.org:some-group/some-project.git
Using LDAP:     yes
Using Omniauth: no

GitLab Shell
Version:        3.0.0
Repositories:   /var/opt/gitlab/git-data/repositories
Hooks:          /opt/gitlab/embedded/service/gitlab-shell/hooks/
Git:            /opt/gitlab/embedded/bin/git