docker: replace maintainer by label in Dockerfiles

The maintainer instruction is deprecated, see:
https://docs.docker.com/engine/reference/builder/#maintainer-deprecated

Also unified the maintainer value.

docker/btcd/Dockerfile

@@ -1,6 +1,6 @@
 FROM golang:1.12-alpine as builder
 
-MAINTAINER Olaoluwa Osuntokun <laolu@lightning.network>
+LABEL maintainer="Olaoluwa Osuntokun <laolu@lightning.engineering>"
 
 # Install build dependencies such as git and glide.
 RUN apk add --no-cache git gcc musl-dev

docker/lnd/Dockerfile

@@ -1,6 +1,6 @@
 FROM golang:1.13-alpine as builder
 
-MAINTAINER Olaoluwa Osuntokun <lightning.engineering>
+LABEL maintainer="Olaoluwa Osuntokun <laolu@lightning.engineering>"
 
 # Force Go to use the cgo based DNS resolver. This is required to ensure DNS
 # queries required to connect to linked containers succeed.

docker/ltcd/Dockerfile

@@ -1,6 +1,6 @@
 FROM golang:1.12-alpine as builder
 
-MAINTAINER Olaoluwa Osuntokun <lightning.engineering>
+LABEL maintainer="Olaoluwa Osuntokun <laolu@lightning.engineering>"
 
 # Grab and install the latest version of roasbeef's fork of ltcd and all
 # related dependencies.