Add Apple M1/ARM-64 support to database image

[marteinn]

The postgis/postgis:12-2.5-alpine container used for DB is currently not supported by arm64/M1. This issue is a reminder to update it as soon as postgis/postgis is compatible with arm64. Read more here: postgis/docker-postgis#216

The current workaround is to use a docker-compose override with another db image, example:

# docker-compose override for using a ARM64 compatible db
version: "3.1"
services:
  db:
    image: gangstead/postgis:13-3.1-arm