Use Debian Buster-based source container image

[jgaskins]

The postgres:11 container image is currently based on Debian Bullseye, which does not have postgres-server-dev-11 in its package repo. Presumably, it used to be based on Buster. This commit ensures we use the Debian release that supports this package.

See: https://packages.debian.org/search?suite=default&section=all&arch=any&searchon=names&keywords=postgresql-server-dev-11