Add internal DNS server if no custom DNS is set (#45)

Fixes #44

Co-authored-by: Lauris BH <lauris@nix.lv>
Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/45
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lafriks@noreply.codeberg.org>
Co-committed-by: Lauris BH <lafriks@noreply.codeberg.org>

Corefile

@@ -0,0 +1,3 @@
+.:53 {
+    forward . /etc/resolv.conf
+}