Configuring gitlab on docker using smtp smarthost

I have deployed gitlab on docker and I want to use a local smarthost (postfix).
I have configured sendmail on the docker host with /etc/mail/

define(`SMART_HOST', `[]')dnl use this host for relaying emails
define(`RELAY_MAILER_ARGS', `TCP $h 25')dnl use smtp port 25
MASQUERADE_AS( use this domain for the sender

then testing from the docker host with “sendmail” works correctly.

but it’s not working from the gitlab container.
I have changed configuration in /etc/gitlab/gitlab.rb and ran gitlab-ctl reconfigure

gitlab_rails['gitlab_email_enabled'] = true
gitlab_rails['smtp_enable'] = true
gitlab_rails['smtp_address'] = ""
gitlab_rails['smtp_port'] = 25
gitlab_rails['smtp_domain'] = ""

The smarthost is running postfix.
Has anyone tried to configure gitlab with a postfix smarthost ? (on docker, if this changes anything ?)

I have replaced my real domain name by in this post.
Thank you for any help.