dockcross-script: Specify --platform flag
dockcross-script: Specify --platform flag

All images built for a linux/amd64 host.

Addresses warning when running an Apple M1:

  WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested

dockcross/dockcross,dockcross/dockcross,dockcross/dockcross,dockcross/dockcross