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.

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