Add openntpd container

Added to Docker example as Swarm mode likes time sync.

Note uses pool.ntp.org at present, on some providers it
should use a different source.

Constraints not enabled as they seem to have an issue;
possibly trying to set before network is up.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

pkg/openntpd/Dockerfile

@@ -0,0 +1,11 @@
+FROM alpine:3.5
+
+RUN \
+  apk update && apk upgrade -a && \
+  apk add --no-cache \
+  openntpd \
+  && true
+
+COPY . .
+
+CMD ["/usr/sbin/ntpd", "-d", "-s"]