cookbook/container: example of building+running docker containers.

This got... long... and complicated.  But I think it's a really good
demonstration of getting redo to do complicated things elegantly.  At
least, I hope it is.

docs/cookbook/container/default.image.do

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+redo-ifchange template.json "$1.layers"
+layers=$(cat "$1.layers")
+
+dir=$3.tmp
+rm -rf "$dir"
+mkdir -p "$dir"
+
+# Build all layers in parallel
+for layer in $layers; do
+	echo "$layer.list.sha256"
+	echo "$layer.layer"
+done | xargs redo-ifchange
+
+ids=
+parent=
+for layer in $layers; do
+	read cid <$layer.list.sha256
+	echo "layer: $cid $layer" >&2
+	
+	# docker seems to order its image tarballs latest-first,
+	# so the base layer is last.  We'll create in order from
+	# base layer to final layer, but create a tarball in the
+	# opposite order.
+	ids="$cid $ids"  # prepend
+
+	mkdir "$dir/$cid"
+	echo "1.0" >$dir/$cid/VERSION
+	./dockjson.py "$layer" "$parent" >$dir/$cid/json
+	ln "$layer.layer" "$dir/$cid/layer.tar"
+	parent=$layer
+done <$1.layers
+
+tar -C "$dir" -cf - $ids >$3
+rm -rf "$dir"