This is a docker image that allows you to setup gitlab using environment variables.

for instance you can setup smtp like this:

GITLAB_SMTP_ENABLE=true
GITLAB_SMTP_ADDRESS="smtp.server"
GITLAB_SMTP_PORT=456
GITLAB_SMTP_USER_NAME="smtp user"
GITLAB_SMTP_PASSWORD="smtp password"
GITLAB_SMTP_DOMAIN="example.com"

Read gitlab documentation:

If you are using an existing docker image gitlab.rb is inside of the config volume. You need to copy it inside the volume. You can do it like this:

docker run -ti framallo/gitlab cp /gitlab.rb /etc/gitlab/gitlab.rb

Last, remember to run gitlab-ctl reconfigure every time you change the config file