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.runlocal.do

@@ -0,0 +1,25 @@
+redo-ifchange "$2.fs"
+
+./need.sh unshare
+
+set +e
+unshare -r chroot "$2" /init >$3
+rv=$?
+if [ "$rv" != 0 ]; then
+	f=/proc/sys/kernel/unprivileged_userns_clone
+	if [ -e "$f" ]; then
+		read v <$f
+		if [ "$v" -eq 0 ]; then
+			echo "Try: echo 1 >$f" >&2
+		fi
+	fi
+
+	f=/proc/sys/kernel/userns_restrict
+	if [ -e "$f" ]; then
+		read v <$f
+		if [ "$v" -ne 0 ]; then
+			echo "Try: echo 0 >$f" >&2
+		fi
+	fi
+fi
+exit "$rv"