Postgis support

[jperelli]

There's an error when trying to upgrade and postgis is installed, I've found the following workaround

docker run -ti --rm -v ./.volumes/db:/var/lib/postgresql --entrypoint="" tianon/postgres-upgrade:14-to-16 bash
root@7acd0925d768:/var/lib/postgresql# apt update && apt install postgresql-14-postgis-3 postgresql-16-postgis-3
[...]
root@7acd0925d768:/var/lib/postgresql# docker-upgrade pg_upgrade

But I was thinking if there's something that can be added as an option when running the image, maybe the entrypoint could have extra packages names as an argument and install them before running, for example

docker run -ti --rm -v ./.volumes/db:/var/lib/postgresql tianon/postgres-upgrade:14-to-16 --packages=postgresql-14-postgis-3,postgresql-16-postgis-3

and that just runs apt update && apt install postgresql-14-postgis-3 postgresql-16-postgis-3 before the upgrade

[tianon]

See #51