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.gz.do

@@ -0,0 +1,13 @@
+redo-ifchange "$2"
+
+# On freebsd, 'gzip --rsyncable' fails but returns 0.
+# We have to detect lack of --rsyncable some other way.
+gzt=$(gzip --rsyncable -c </dev/null 2>/dev/null | wc -c)
+if [ "$gzt" -gt 0 ]; then
+	# when available, --rsyncable makes compressed
+	# files much more efficient to rsync when they
+	# change slightly.
+	gzip --rsyncable -c <$2 >$3
+else
+	gzip -c <$2 >$3
+fi