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.yml

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