Caddy x GCS reverse proxy via storage.googleapis.com

Caddyfile

# requires caddyserver/cache-handler module

{
	order cache before rewrite
	cache
	log default {
		output stdout
		# http.handlers.cache logs the entire contents of request
		# bodies at info level(??!)
		exclude http.handlers.cache
	}
	log gcs {
		output stdout
		level debug
		include http.handlers.reverse_proxy
	}
}

my-site.example {
	log stdout

	@read method GET HEAD OPTIONS
	handle @read {
		cache
		rewrite * /bucket-name{uri}
		reverse_proxy https://storage.googleapis.com {
			header_up Host {upstream_hostport}
		}
	}

	handle {
		header Allow "GET, HEAD, OPTIONS"
		respond 405 {
			body "405 Method Not Allowed"
		}
	}
}