$PWD/Caddyfile must not be a file that exists on your host machine then. If you’re saying your Caddyfile is on your host machine at /usr/share/hassio/share/caddy/Caddyfile, then you should mount it like this:
- /usr/share/hassio/share/caddy/Caddyfile:/etc/caddy/Caddyfile
The part to the right of the : is the path inside the container, the part to the left of the : is the path on the host machine.