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