fix: persist config across restarts

- root cause: Technitium image ships /etc/dns as a real directory, so the
  build-time 'ln -sf /data/dns /etc/dns' created the symlink INSIDE it
  (/etc/dns/dns) instead of replacing it. Technitium kept writing to the
  ephemeral container directory and lost everything on restart.
- fix: add entrypoint.sh that runs at container start (after HA mounts
  volumes), removes the real /etc/dns dir, and symlinks it to /config/dns
  (the persistent addon_config mount). Seeds defaults on first run only.
- bump version to 15.4.2

technitium_dns/Dockerfile

@@ -1,12 +1,10 @@
 ARG BUILD_FROM=ghcr.io/hassio-addons/base:latest
 FROM technitium/dns-server:latest
 
-ENV DATA_DIR="/data/dns" \
-    DNS_SERVER_WEB_SERVICE_HTTPS_PORT=53443 \
+ENV DNS_SERVER_WEB_SERVICE_HTTPS_PORT=53443 \
     DNS_SERVER_WEB_SERVICE_ENABLE_HTTPS=false
 
-RUN mkdir -p /data/dns && \
-    ln -sf /data/dns /etc/dns
+COPY entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
 
-ENTRYPOINT []
-CMD ["dotnet", "DnsServerApp.dll"]
+ENTRYPOINT ["/entrypoint.sh"]