Danger: Pin ruby base image to Debian buster

Until https://github.com/nodesource/distributions/issues/1249 is resolved.

The upstream upgrade to Debian Bullseye broke node.js package dependencies:

dpkg: dependency problems prevent configuration of nodejs:
 nodejs depends on python-minimal; however:
  Package python-minimal is not installed.
dpkg: error processing package nodejs (--install):
 dependency problems - leaving unconfigured
Errors were encountered while processing:
 nodejs