Update Dockerfile maintainers

Dockerfile

 FROM ubuntu:16.04
-MAINTAINER hrvoje.marjanovic@gmail.com
+LABEL authors.maintainer "Grzegorz Bizon <grzegorz@gitlab.com>"
+LABEL authors.contributor "Hrvoje Marjanovic <hrvoje.marjanovic@gmail.com>"
 ENV DEBIAN_FRONTEND noninteractive
 RUN apt-get update
 RUN apt-get -y install curl git-core software-properties-common python-software-properties
+
 # This PPA contains an up-to-date version of Go
 RUN apt-add-repository -y ppa:ubuntu-lxc/lxd-stable
 RUN apt-get update
-
 # install essentials
 RUN apt-get -y install build-essential
 RUN apt-get install -y -q git