do not install recomended packets

Dockerfile

 RUN mv -v /usr/share/postgresql/9.6/pg_hba.conf.sample /usr/share/postgresql/ \
               && ln -sv ../pg_hba.conf.sample /usr/share/postgresql/$PG_MAJOR/
 
-RUN apt-get update && apt-get -y install\
+RUN apt-get update && apt-get -y --no-install-recommends install\
         cron\
         python3\
         python3-pip\