hardcoded PGMAJOR for compatibility

Dockerfile

@@ -1,6 +1,6 @@
 FROM robinthoni/postgres-multiarch:9.6
 
-RUN mv -v /usr/share/postgresql/$PG_MAJOR/pg_hba.conf.sample /usr/share/postgresql/ \
+RUN mv -v /usr/share/postgresql/9.6/pg_hba.conf.sample /usr/share/postgresql/ \
               && ln -sv ../pg_hba.conf.sample /usr/share/postgresql/$PG_MAJOR/
 
 RUN apt-get update && apt-get -y install\