make image builds amd64 only, even on an arm64 machine #7

The dev image target can only build amd64. make image runs docker build --platform=linux/amd64 (Makefile, the image target that pushes to ttl.sh), so a developer on an arm64 machine gets an amd64 image and cannot test the proxy natively, and there is no way to build an arm64 dev image with it.

Once the Dockerfile cross-compiles (#6), the fixed flag is the only thing in the way. Taking the platform from a variable that defaults to the host, or to linux/amd64 for the current behaviour, would be enough.