test: Use minidns in the Docker tests
The integration tests depend on having a DNS server that resolves "localhost", which is unfortunate but currently unavoidable given glibc's limitations ($HOSTALIASES only works on DNS-level aliases, and does not do lookups in /etc/hosts). Even under docker, this makes the tests depend on the DNS server, and whether it resolves localhost or not. In order to make the docker tests more hermetic and isolated from the environment, this patch introduces a docker entrypoint that, within the container, will launch minidns and override /etc/resolv.conf to use it. This guarantees that the tests will be able to resolve localhost, and also avoid accidental reliance on external DNS zones.
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