Yes, you're right, you can define only one bootstrap DNS server.

It would make sense to allow multiple bootstrap DNS server. Since we allow also the definition of IP addresses (for DoH and DoT):

bootstrapDns:
      upstream: tcp-tls:dns.example.com
      ips:
      - 123.123.123.123

we can't just change upstream to a list structure (or comma separated string). I think, we need more complex structure and we should also support current config (to avoid breaking changes).

Started looking into this.

For the config, we should be able to transparently support the old format with something like:

bootstrapDns:
  - upstream: tcp-tls:dns1.example.com
    ips:
      - 123.123.123.123
  - upstream: tcp-tls:dns2.example.com
    ips:
      - 456.456.456.456

For the implementation, there's a little refactoring needed, but I don't foresee anything major. The awkward part is that with multiple bootstrap upstreams, we want to race them using ParallelBestResolver, but that resolver uses the Bootstrap to test the upstreams it uses. So it's a dependency loop.
There's code to bypass those checks, only used by tests at the moment, so my plan is to also use that for construction as part of the Bootstrap.
Hopefully I'll make a PR soon!

Thanks for this, @0xERR0R and @ThinkChaos. Any idea, when it will be included in images on dockerhub/GCR?

The development tag has this already (CI had failed before, but I retried it), so if you want to try it out before a full release you can play with that.

Nice, thanks again.