Dockerfile: run apt-get upgrade

[pjonsson]

What this PR does

This installs the security updates
from the OS vendor in the Docker
image.

Test me

Build the Docker image and see that it still builds.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.