removed unused etc files

postfix/Dockerfile

@@ -4,13 +4,13 @@ FROM ubuntu:20.04
 ENV TZ Europe/Berlin 
 RUN <<EOF
 apt-get update
-apt-get install -y postfix
+apt-get install -y postfix bind9-dnsutils
 # cleanup
 apt-get autoclean
 rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* ~/.cache ~/.npm
-echo zntrl.de >>/etc/mailname
 EOF
 COPY etc/postfix/ /etc/postfix/
+RUN cp /etc/resolv.conf /var/spool/postfix/etc/resolv.conf
 COPY --chmod=0775 entrypoint.sh /entrypoint.sh
 EXPOSE 25
 VOLUME /var/spool/postfix