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/debdownload.fs.do

@@ -0,0 +1,14 @@
+exec >&2
+fs=${1%.fs}
+
+# let's *not* delete this directory; it's okay if previously-downloaded
+# excess packages hang around in case we need them later.
+#rm -rf "$fs"
+mkdir -p "$fs"
+redo-ifchange debootstrap.options
+debootstrap \
+	--download-only \
+	--keep-debootstrap-dir \
+	$(cat debootstrap.options) \
+	"$fs"
+redo-ifchange "$fs/debootstrap/debootstrap.log"