Split build process to allow multi arch builds

[DuckThom]

Note: I've already pushed the image to https://hub.docker.com/r/elzekool/pma-autoconfig as elzekool/pma-autoconfig:latest

I've used Docker buildx on my mac to create a multiarch image:

1. docker buildx create --driver docker-container --name local --use unix:///var/run/docker.sock
2. docker buildx inspect --bootstrap local and verify that it shows at least linux/amd64 and linux/arm64 as available platforms
3. docker buildx build --platform linux/arm64,linux/amd64 -t elzekool/pma-autoconfig:latest . to build and check if the build is working. add --push to push the image to docker hub