Limited platforms for docker image

[hsiliev]

Running OpenArchiefbeheer on Apple M1-4 produces warnings like:

 ! openarchiefbeheer-web The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested               0.0s
 ! openarchiefbeheer-celery The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested            0.0s
 ! openarchiefbeheer-celery-beat The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested       0.0s

If I'm not mistaked, just adding --platform linux/amd64 while building the Docker image should solve this.

Potentially adding more platforms would also allow use on ARM VMs in clouds like Hetzner