README: fix Build Your Own IB example

OpenWrt switched to using ZSTD compressed tarballs a while ago, so using
the current example DOWNLOAD_FILE arg will result in the built container
not being able to find any files since that tarball is missing.

So, update to also look for .tar.zst as well.

Signed-off-by: Robert Marko <robimarko@gmail.com>

Author: robimarko

README.md

@@ -149,7 +149,7 @@ If you wan to create your own container you can use the `Dockerfile`. You can se
 ```shell
 docker build \
     --build-arg TARGET=x86/64 \
-    --build-arg DOWNLOAD_FILE="imagebuilder-.*x86_64.tar.xz" \
+    --build-arg DOWNLOAD_FILE="imagebuilder-.*x86_64.tar.[xz|zst]" \
     --build-arg FILE_HOST=downloads.openwrt.org \
     --build-arg VERSION_PATH=snapshots \
     -t openwrt/x86_64 .