fixed postfix configuration

postfix/Dockerfile

@@ -1,8 +1,16 @@
-# syntax=docker/dockerfile:1
-FROM alpine:latest
+# syntax=docker.io/docker/dockerfile:1.5.2
+FROM ubuntu:20.04
 # install apt packages
 ENV TZ Europe/Berlin 
-RUN apk add --no-cache postfix spamassassin rsyslog logrotate xz
+RUN <<EOF
+apt-get update
+apt-get install -y postfix
+# 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/
 COPY --chmod=0775 entrypoint.sh /entrypoint.sh
 EXPOSE 25
 VOLUME /var/spool/postfix