Generate certificates on first run

[darkdragon-001]

1. It is not a good idea to store keys in the container
2. Certificats expire, so it is very likely that certificates are unusable when one wants to try out the container

As noted in /etc/raddb/certs/README, bootstrap scripts is run on first start automatically. So I suggest to not run the bootstrap part in the Dockerfile but instead let it handle freeradius automatically. In addition, I suggest to add a volume mount for certs folder which would be populated when first starting docker compose.