Upgrade the base distribution
Our Docker image is layered on top of the upstream GitLab CE image (currently 12.2.12-ce.0, itself based on Ubuntu 16.04).
It's against the recommended Docker practices to perform a base image upgrade because it can create dependency issues with some essential packages. In practice, if the base distribution image is not too old, it works. The resulting image is expected not to start if it doesn't work.