Use official postgis image and upgrade to latest version

[joefutrelle]

https://registry.hub.docker.com/r/postgis/postgis/

on the deployment side this will need to be accomplished with database backup / restore

[joefutrelle]

Parameterize the postgis image name/tag so that deployments can choose to retain older (non EOL) version so as to avoid database migration

[mike-kaimika]

@joefutrelle I had created a pull request, #373, based on what we spoke about last before I realized there was an existing issue in GitHub. Let me know if you want to have the pull request associated with a separate ticket, since its only for swapping to the official image and not updating the PostgreSQL version.

Also noting it here, as I did in the PR itself, that after the update, a manual command has to be run to install/update/fix PostGIS

docker-compose exec postgres update-postgis.sh