loop: introduce docker integration (#46)

* Introduced docker integration
* Added Dockerfile to build loopd and loop with no dependencies.
* Added instructions to build and run docker image.

* Fixed documentation on mapping of ~/.lnd
* Would have caused problems because ~/.loop wasn't mapped
* Now map entire home directory

* Updated DOCKER.md example to just map ~/.lnd and ~/.loop.
* Removed mapping of home directory - one of the benefits of docker is being explicit about the resources the container uses so I felt bad mapping the entire home directory when only two known folders were used.

DOCKER.md

@@ -0,0 +1,83 @@
+# Docker
+
+
+## Prerequisites
+
+The only prerequisites are:
+1. This repo, and
+2. Docker
+
+Building the docker image pulls in all the dev dependencies to build loop within the image itself. Having a `go` development environment is not required.
+
+
+## Building the Docker Image
+
+The docker image can be built using this command within the `loop` directory:
+```
+docker build --tag loop .
+```
+This command pulls down a `go` build container, builds `loop` and `loopd` executables, then publishes those binaries to a fresh, smaller image, and marks that image with the tag 'loop'.
+
+
+## Running the Docker Image
+
+The docker image contains:
+* The binary `loopd`, at `/go/bin/loopd`
+* The binary `loop`, at `/go/bin/loop`
+
+Docker is very flexible so you can use that information however you choose. This guide isn't meant to be prescriptive.
+
+
+### Example: Running loopd
+
+One way of running `loopd` is 
+```
+docker run --rm -it --name loopd -v $HOME/.lnd:/root/.lnd -v $HOME/.loop:/root/.loop loop:latest loopd --network=testnet --lnd.host <my-lnd-ip-address>:10009
+```
+
+Things to note from this docker command:
+* You can stop the server with Control-C, and it'll clean up the associated stopped container automatically.
+* The name of the running container is 'loopd' (which you may need to know to run the `loop` command).
+* The '.lnd' directory in your home directory is mapped into the container, and `loopd` will look for your tls.cert and macaroon in the default locations. If this isn't appropriate for your case you can map whatever directories you choose and override where `loopd` looks for them using additional command-line parameters.
+* The '.loop' directory in your home directory is mapped into the container, and `loopd` will use that directory to store some state.
+* You probably need to specify your LND server host and port explicitly, since by default `loopd` looks for it on localhost and there is no LND server on localhost within the container.
+* No ports are mapped, so it's not possible to connect to the running `loopd` from outside the container. (This is deliberate. You can map ports 8081 and 11010 to connect from outside the container if you choose.)
+
+
+### Example: Running loop
+
+If you're using the example above to run `loopd`, you can then run the `loop` command inside that running container to execute loops. One way would be:
+```
+docker exec -it loopd loop out --channel <channel-id-you-want-to-use> --amt <amount-you-want-to-loop-out>
+```
+
+Things to note about this docker command:
+* `docker exec` runs a command on an already-running container. In this case `docker exec loopd` says effectively 'run the rest of this command-line as a command on the already-running container 'loopd'.
+* The `-it` flags tell docker to run the command interatively and act like it's using a terminal. This helps with commands that do more than just write to stdout.
+* The remainder `loop out --channel <channel-id-you-want-to-use> --amt <amount-you-want-to-loop-out>` is the actual loop command you want to run. All the regular `loop` documentation applies to this bit.
+
+
+### A Handy Script
+
+If you're using the example above to run `loopd`, creating a script can simplify running `loop`.
+
+Create a file with the following contents:
+```
+#!/usr/bin/env bash
+TERMINAL_FLAGS=
+if [ -t 1 ] ; then
+        TERMINAL_FLAGS="-it"
+fi
+
+docker exec $TERMINAL_FLAGS loopd loop "${@}"
+```
+Call this script 'loop', put it somewhere in your $PATH, and make it executable. Then you can just run commands like:
+```
+loop out --channel <channel-id-you-want-to-use> --amt <amount-you-want-to-loop-out>
+```
+without having to remember (or use) the docker part explicitly.
+
+
+## Caveats
+
+Running `loopd` the way shown above won't restart `loopd` if it is stopped or if the computer is restarted. You may want to investigate running the 'loop' container at startup, or when your LND server starts. (For example, `docker` has restart options, or grouping of containers via `docker-compose`.)

Dockerfile

@@ -0,0 +1,33 @@
+FROM golang:1.12-alpine as builder
+
+# Copy in the local repository to build from.
+COPY . /go/src/github.com/lightningnetwork/loop
+
+# Force Go to use the cgo based DNS resolver. This is required to ensure DNS
+# queries required to connect to linked containers succeed.
+ENV GODEBUG netdns=cgo
+
+# Explicitly turn on the use of modules (until this becomes the default).
+ENV GO111MODULE on
+
+# Install dependencies and install/build lnd.
+RUN apk add --no-cache --update alpine-sdk \
+    git \
+    make \
+&&  cd /go/src/github.com/lightningnetwork/loop/cmd \
+&&  go install ./...
+
+# Start a new, final image to reduce size.
+FROM alpine as final
+
+# Expose lnd ports (server, rpc).
+EXPOSE 8081 11010
+
+# Copy the binaries and entrypoint from the builder image.
+COPY --from=builder /go/bin/loopd /bin/
+COPY --from=builder /go/bin/loop /bin/
+
+# Add bash.
+RUN apk add --no-cache \
+    bash \
+    ca-certificates