rootfs/bin/caddy: Replace bin/caddy with distribution binary v0.8.3
rootfs/bin/caddy: Replace bin/caddy with distribution binary v0.8.3

[Expermentally] replace rootfs/bin/caddy with the binary distributed
at Caddy's github releases page:
https://github.com/mholt/caddy/releases/download/v0.8.3/caddy_linux_amd64.tar.gz

Formerly bin/caddy was locally cross-compiled.

joshix/caddybox