ref[Dockerfile]:Name builder image; refactor Maintainer (deprecated)

Dockerfile

 ### Build Image
-FROM golang:1.14-buster
-MAINTAINER Christian Pointner <equinox@helsinki.at>
+FROM golang:1.14-buster AS builder
+LABEL maintainer="Christian Pointner <equinox@helsinki.at>"
 
 RUN set -e \
   && apt-get update -q \
@@ -20,7 +20,7 @@ RUN make
 
 ### APP Image
 FROM debian:buster
-MAINTAINER Christian Pointner <equinox@helsinki.at>
+LABEL maintainer="Christian Pointner <equinox@helsinki.at>"
 
 RUN set -e \
   && apt-get update -q \
@@ -33,7 +33,7 @@ RUN set -e \
   && chown 1000:1000 /srv/audio /srv/import-temp
 
 COPY contrib/for-docker.yaml /etc/aura/tank.yaml
-COPY --from=0 /usr/src/tank /usr/local/bin/tank
+COPY --from=builder /usr/src/tank /usr/local/bin/tank
 
 VOLUME /srv/audio
 VOLUME /srv/import-temp