Docker images - no postgis/mysql image for linux/arm64 (e.g. Apple M1)

* There's currently no linux/arm64 images for postgis nor mysql
* For MySQL we can switch to [`bitnami/mysql`](https://hub.docker.com/r/bitnami/mysql/tags), but there's no alternative for postgis that I can find at the moment. Official postgis ARM image [still in the works](https://github.com/postgis/docker-postgis/issues/216)