I realise this is quite a specific post; this is mainly for posterity.
I have a Synology box running this docker image for GitLab. I also have some interest in configuring GitLab to receive emails from a specified mailbox, and create new issues upon receipt of an email.
This is possible with the docker image, using variables such as
IMAP_USER as described in the gitlab docker page.
When this is all set up, the docker image fails to start its mail client. You will need to log in to the docker image, by first running
docker ps. This will allow you to obtain the
CONTAINER_ID of the docker image.
This will allow you to obtain a shell for the docker image, by typing
docker exec -it CONTAINER_ID bash.
Then, on the docker image, run
sudo -u git -H RAILS_ENV=production bin/mail_room start. This will start the
mail_room, and if configured correctly, your docker image should start polling the configured IMAP server.