The entrypoint script was broken as in that multiple backends, as by
default, would result in the daemon breaking as the configuration became
invalid. This was because only the first entry for upstream DNS
providers was properly formatted with a `udp:` as prefix.

This patch fixes the `sed` command to add a proper prefix to all entires
in the config.

The error message caused was:

```
One of the upstreams has not a (udp|tcp|tcp-tls) prefix e.g. udp:1.1.1.1:53
```