made postfix Alpine based

postfix/Dockerfile.ubuntu

@@ -0,0 +1,13 @@
+# syntax=docker/dockerfile:1.3-labs
+FROM ubuntu:20.04
+# install apt packages
+RUN <<EOF 
+apt-get update -y
+apt-get install -y postfix spampd 
+apt-get autoclean
+rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* ~/.cache ~/.npm
+EOF
+WORKDIR /root
+EXPOSE 25
+VOLUME /var/spool/postfix
+ENTRYPOINT ["/usr/sbin/postmulti -i %i -p start"]