Updated deprecated Dockerfile field.

Dockerfile

 FROM python:3.7.7-buster
 
-MAINTAINER David Trattnig <david.trattnig@subsquare.at>
+LABEL maintainer="David Trattnig <david.trattnig@subsquare.at>"
 
 RUN set -ex \
   && rm -f /usr/bin/python && ln -s /usr/local/bin/python /usr/bin/python \