Revert "fix: symlink /etc/dns to /config/dns at build time, drop entrypoint"

This reverts commit ed54778a0d.

technitium_dns/entrypoint.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+set -e
+
+# Technitium ships /etc/dns as a real directory in the image. We need it
+# to point at the persistent HA add-on config volume instead. Remove the
+# directory and replace with a symlink at runtime (after HA mounts volumes).
+
+DATA="/config/dns"
+
+mkdir -p "$DATA"
+
+if [ -d /etc/dns ] && [ ! -L /etc/dns ]; then
+    # Seed: copy any existing default config into the volume on first run
+    if [ -z "$(ls -A "$DATA" 2>/dev/null)" ]; then
+        cp -a /etc/dns/. "$DATA"/
+    fi
+    rm -rf /etc/dns
+fi
+
+ln -sf "$DATA" /etc/dns
+
+exec dotnet DnsServerApp.dll