Raspbian is unfortunately still a 32-bit OS for the moment (because older Pi devices), so I imagine the “arm64” option is out. I don’t see an ARMv8 option on the Caddy download page.
I’m not 100% sure if the OS matters; try arm64 anyway. I could be wrong, but I think the build architecture is irrelevant to the operating system. It’s still Linux, so I’d expect all the syscalls to work.