nix: Migrate scripts

They're a bit slower to start :(.
Hopefully this is something that flakes will help with,
otherwise I'll find another way.

config/nix/scripts/docker-image-childs

@@ -0,0 +1,13 @@
+#!/usr/bin/env nix-shell
+#! nix-shell -i bash --pure
+#! nix-shell -p bash docker gnugrep coreutils
+
+# Find the dependent child image from an image
+
+parent="$1"
+
+# From https://stackoverflow.com/a/41634462
+for i in $(docker images -q)
+do
+    docker history "$i" | grep -q "$parent" && echo "$i"
+done | grep -v "$parent" | sort -u