run-dns-server

Before running any acceptance tests for the DNS provider, start a DNS server:

Check whether the DNS server is already alive by querying its locally-mapped port:
```
dig @127.0.0.1 -p 15353 +short ns.example.com
```
Expected command output is "127.0.0.1."

If the DNS server is not already alive, then start the DNS server by running this command in the background:

docker run --privileged --cgroupns=host -d --tmpfs /tmp --tmpfs /run \
 -v /sys/fs/cgroup:/sys/fs/cgroup:rw \
 -v /etc/localtime:/etc/localtime:ro \
 -v $PWD/internal/provider/testdata/named.conf.kerberos:/etc/named.conf:ro \
 -p 127.0.0.1:15353:53 \
 -p 127.0.0.1:15353:53/udp \
 --rm --name ns --hostname ns.example.com ns

After starting the DNS server, verify that the DNS server is alive by querying its locally-mapped port:
```
dig @127.0.0.1 -p 15353 +short ns.example.com
```
Expected command output is "127.0.0.1." Display no output unless there is a problem.
If there is a problem, do not run the acceptance tests.

Add these environment variables to all go test commands for acceptance tests in the DNS provider:

DNS_UPDATE_SERVER=127.0.0.1
DNS_UPDATE_PORT=15353

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