[BUG] Document USE_COREDNS environment variable

[viktorlindgren]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

Missing documentation of USE_COREDNS environment variable in the readme file

cat /etc/s6-overlay/s6-rc.d/svc-coredns/run 
#!/usr/bin/with-contenv bash

if netstat -apn | grep -q ":53 "; then
    USE_COREDNS="false"
fi

if [[ ${USE_COREDNS} == "false" ]]; then
    s6-notifyoncheck -d -n 300 -w 1000 -c "echo **** Disabling CoreDNS ****" \
        sleep infinity
else
    if grep -q "health" /config/coredns/Corefile; then
        exec \
          s6-notifyoncheck -d -n 300 -w 1000 -c "redirfd -w 1 /dev/null curl -s http://localhost:8080/health" \
              cd /config/coredns \
              /usr/bin/coredns -dns.port=53
    else
        exec \
          s6-notifyoncheck -d -n 300 -w 1000 -c "nc -zu localhost 53" \
              cd /config/coredns \
              /usr/bin/coredns -dns.port=53
    fi
fi

### Expected Behavior

Documentation to include information about USE_COREDNS environment variable

Example:
https://github.com/linuxserver/docker-wireguard/commit/34272b140cf9006241ac978ae47bb51311f8f640

If the user use external DNS server, like 8.8.8.8, then the internal dns server is not needed and the service can be disabled.

### Steps To Reproduce

Documentation https://github.com/linuxserver/docker-wireguard/blob/master/.github/CONTRIBUTING.md say create issue instead of a pull request - this one.

### Environment

```markdown
- OS: Ubuntu 24.04
- How docker service was installed: distro's packagemanager(apt)

CPU architecture

x86-64

Docker creation

See expected behavior note

Container logs

Not relevant

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[j0nnymoe]

Already explained here #351 (comment) - it's purposely kept undocumented.