xcaddy build has no way to cache dependency files

I hope I can execute `xcaddy build --download-dependency --with ...` and then execute `xcaddy build --with ...` separately, so that the Dockerfile can cache the dependency layer. Dockerfile: ``` FROM caddy:builder AS builder COPY ./caddy-plugin /app/caddy-plugin RUN xcaddy build --download-dependency --with example.com/custom-plugin=/app/caddy-plugin/custom-plugin RUN xcaddy build --with example.com/custom-plugin=/app/caddy-plugin/custom-plugin FROM caddy COPY --from=builder /usr/bin/caddy /usr/bin/caddy ```