Upgrade base image to Debian 10 ("buster")

[joakime]

The current image is based on Debian 9 ("stretch") which has now been superseded by Debian 10 ("buster").

From https://www.debian.org/releases/stretch/

Debian 9 has been superseded by Debian 10 (buster). Security updates have been discontinued as of July 6th, 2020.

We should update to Debian 10 ("buster") to get security updates, and also to stay current with openjdk-11 which is no longer receiving updates on stretch.

• Stretch Backports is still on openjdk 11.0.6 - https://packages.debian.org/stretch-backports/openjdk-11-jre-headless
• Buster Stable is on openjdk 11.0.8 - https://packages.debian.org/buster/openjdk-11-jdk-headless