DERP container cannot read certs if file permissions incorrect

When certificate files in /root/derper-data lack correct permissions (fullchain.pem not 644 or privkey.pem not 600), DERP container cannot read certificates causing SSL handshake failure, manifesting as curl hanging on Client Hello

Set chmod 644 on derp.example.com.crt (fullchain) and chmod 600 on derp.example.com.key (privkey) in /root/derper-data. Verify container can read by checking curl -Iv https://derp.example.com:3443 returns HTTP/1.1 200 OK with SSL certificate verify ok