always build linux/amd64 images

you can also specify this (maybe?) in challenge.yaml and definitely in
your Dockerfile, `FROM --platform=linux/amd64 ubuntu:22.04 AS build`,
but cmon... who's ever building a non x86 chal?

rcds/challenge/docker.py

@@ -209,6 +209,7 @@ def _build(self) -> None:
             buildargs=self.buildargs,
             pull=True,
             rm=True,
+            platform="linux/amd64",
         )
         self.project.docker_client.images.push(
             self.image, tag=self.content_hash, auth_config=self.manager._auth_config