docker+docs: move dev Dockerfile to root, document dev build

To finally end the discussion what Dockerfile should be used for what
and whether we should build from local source or check out from git, we
place both Dockerfiles next to each other and explicitly document their
purpose.

docker/docker-compose.ltc.yml

@@ -24,7 +24,7 @@ services:
       container_name: lnd_ltc
       build:
         context: ../
-        dockerfile: docker/lnd/Dockerfile
+        dockerfile: dev.Dockerfile
       environment:
         - RPCUSER
         - RPCPASS

docker/docker-compose.yml

@@ -24,7 +24,7 @@ services:
       container_name: lnd
       build:
         context: ../
-        dockerfile: docker/lnd/Dockerfile
+        dockerfile: dev.Dockerfile
       environment:
         - RPCUSER
         - RPCPASS

docs/DOCKER.md

@@ -1,9 +1,25 @@
 # Docker Instructions
 
+There are two flavors of Dockerfiles available:
+ - `Dockerfile`: Used for production builds. Checks out the source code from
+   GitHub during build. The build argument `--build-arg checkout=v0.x.x-beta`
+   can be used to specify what git tag or commit to check out before building.
+ - `dev.Dockerfile` Used for development or testing builds. Uses the local code
+   when building and allows local changes to be tested more easily.
+
 ## Development/testing
 
-For development or testing, or to spin up a `btcd` backend alongside `lnd`,
-check out the documentation at [docker/README.md](../docker/README.md).
+To build a standalone development image from the local source directory, use the
+following command:
+
+```
+$ docker build --tag=myrepository/lnd-dev -f dev.Dockerfile .
+```
+
+There is also a `docker-compose` setup available for development or testing that
+spins up a `btcd` backend alongside `lnd`. Check out the documentation at
+[docker/README.md](../docker/README.md) to learn more about how to use that
+setup to create a small local Lightning Network.
 
 ## Production