Support older PostGIS versions
Support older PostGIS versions

Add the PGDG archive APT repository to support the installation of previous
PostGIS versions. Also, upgrade the underlying Debian install, as the
"Official Images" stop providing updates for older PostgreSQL releases (e.g.,
https://github.com/docker-library/postgres/commit/7f5f6da5a1976bfd2c6d989e20cef080d0d9c68f)

See: https://www.postgresql.org/about/news/announcing-apt-archivepostgresqlorg-2024/

azavea/docker-postgis