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/simple.fs.do

@@ -0,0 +1,25 @@
+exec >&2
+
+# We can pull in the 'hello' binary built in an earlier
+# example.  Notice that it's safe to have dependencies
+# that cross directory boundaries, even when we're building
+# both of those directories in parallel.
+FILES="
+	/bin/sh
+	../hello/hello
+"
+if [ -x /bin/busybox ]; then
+	# Optional, except for runkvm
+	FILES="$FILES /bin/busybox"
+else
+	redo-ifcreate /bin/busybox
+fi
+redo-ifchange $FILES
+
+fs=${1%.fs}
+rm -rf "$fs"
+mkdir -p "$fs/bin"
+cp $FILES "$fs/bin/"
+ln -s bin/hello "$fs/init"
+
+redo-ifchange "$fs/bin/sh"