install: store gitlab application logs at `/var/log/gitlab/gitlab/`
install: store gitlab-shell logs at `/var/log/gitlab/gitlab-shell/`
install: store gitlab vhost access logs at `/var/log/gitlab/nginx/`
install: store supervisor logs at `/var/log/gitlab/supervisor/`
dockerfile: added volume mount instruction for `/var/log/gitlab`
changelog: updated
Merge branch 'logs-volume'

Closes #40