We update our Docker image for security releases and other library updates. These updates are not picked up in admin/upgrade

To update

SSH into your web server using putty or your favorite SSH client.

cd /var/discourse
git pull
./launcher rebuild app

If you are running low on disk space (check with df) try clearing up old images using:

./launcher cleanup

If you cannot upgrade the container because Docker itself is out of date, upgrade Docker first. To upgrade Docker itself on Ubuntu:

apt-get update
apt-get dist-upgrade
apt-get upgrade lxc-docker

Source: https://meta.discourse.org/t/how-do-i-update-my-docker-image-to-latest/23325