PostGIS Docker image: switch to using use official postgis images

[willfurnass]

The one we're using at present has lots of known vulnerabilities in the OS packages; compare

sudo trivy image --severity HIGH,CRITICAL --ignore-unfixed mdillon/postgis:11-alpine

with

sudo trivy image --severity HIGH,CRITICAL --ignore-unfixed docker pull postgis/postgis:11-2.5-alpine
sudo trivy image --severity HIGH,CRITICAL --ignore-unfixed docker pull postgis/postgis:11-3.0-alpine

See https://registry.hub.docker.com/r/postgis/postgis for more info on the official postgis images.

Could also use the opportunity to switch to PostgreSQL 12 or 13.

[griff-rees]

Hey I've got a potential fix: griff-rees@72cd3dc can set up a pull request if you like. I'm pretty sure this comes out of this issue: postgis/docker-postgis#143

[griff-rees]

Just got it working on PostgreSQL 13: griff-rees@4bd3708. Might require an initial purge of previous volumes though: https://cookiecutter-django.readthedocs.io/en/latest/troubleshooting.html#docker-postgres-authentication-failed