do not remove files in /etc/postfix

postfix/Dockerfile

     apt-get install -y postfix postfix-pgsql postfix-ldap rsyslog &&\
     apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
-RUN rm -rf /etc/postfix/*
-
 RUN rm -rf /var/log/* &&\
     mkfifo /var/log/mail.info &&\
     ln -s /dev/null /var/log/mail.log &&\