lnd+docker: add "Production" Docker Image

This commit satisfies the auto-build functionality desired in #722 without changing anything in the docker folder. This allows development and testing to continue to build and use images locally, but external users get a very lean Docker image they can use. The size difference is appreciable.

REPOSITORY                     TAG                 IMAGE ID            CREATED             SIZE
lnd                            latest              3634dcf5df68        17 seconds ago      1.19GB
lnd-single                     latest              b295cc248447        30 minutes ago      56MB
Right before merging this PR, an Automated Build repository should be configured on the appropriate account (lightningnetwork?) on Docker Hub with the name of lnd in order to match documentation.

I see there are already some repositories created, however I think best practice for Docker is to have a single repository and different tags for different architectures. The included image is based off of Alpine, but in the future we could extend into multiple Dockerfiles (example: ARM support, or being Ubuntu based).

So in this proposed scenario, lightningnetwork/lnd:latest would be alpine, but lightningnetwork/lnd:ubuntu would be an ubuntu-based image, for example.

(With thanks to @alingenhag for https://github.com/f-u-z-z-l-e/docker-lnd/ from which I borrowed heavily)

README.md

@@ -1,8 +1,8 @@
 ## Lightning Network Daemon
 
-[![Build Status](http://img.shields.io/travis/lightningnetwork/lnd.svg)](https://travis-ci.org/lightningnetwork/lnd) 
-[![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/lightningnetwork/lnd/blob/master/LICENSE) 
-[![Irc](https://img.shields.io/badge/chat-on%20freenode-brightgreen.svg)](https://webchat.freenode.net/?channels=lnd) 
+[![Build Status](http://img.shields.io/travis/lightningnetwork/lnd.svg)](https://travis-ci.org/lightningnetwork/lnd)
+[![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/lightningnetwork/lnd/blob/master/LICENSE)
+[![Irc](https://img.shields.io/badge/chat-on%20freenode-brightgreen.svg)](https://webchat.freenode.net/?channels=lnd)
 [![Godoc](https://godoc.org/github.com/lightningnetwork/lnd?status.svg)](https://godoc.org/github.com/lightningnetwork/lnd)
 
 <img src="logo.png">
@@ -15,13 +15,13 @@ full-node), [`bitcoind`](https://github.com/bitcoin/bitcoin), and
 [`neutrino`](https://github.com/lightninglabs/neutrino) (a new experimental light client). The project's codebase uses the
 [btcsuite](https://github.com/btcsuite/) set of Bitcoin libraries, and also
 exports a large set of isolated re-usable Lightning Network related libraries
-within it.  In the current state `lnd` is capable of: 
+within it.  In the current state `lnd` is capable of:
 * Creating channels.
 * Closing channels.
 * Completely managing all channel states (including the exceptional ones!).
 * Maintaining a fully authenticated+validated channel graph.
 * Performing path finding within the network, passively forwarding incoming payments.
-* Sending outgoing [onion-encrypted payments](https://github.com/lightningnetwork/lightning-onion) 
+* Sending outgoing [onion-encrypted payments](https://github.com/lightningnetwork/lightning-onion)
 through the network.
 * Updating advertised fee schedules.
 * Automatic channel management ([`autopilot`](https://github.com/lightningnetwork/lnd/tree/master/autopilot)).
@@ -66,6 +66,9 @@ discuss various aspects of `lnd` and also Lightning in general.
 ## Installation
   In order to build from source, please see [the installation
   instructions](docs/INSTALL.md).
+
+## Docker
+  To run lnd from Docker, please see the main [Docker instructions](docs/DOCKER.md)
   
 ## IRC
   * irc.freenode.net