Added systemd unit file

Makefile

-all:
+all: postfix-service
 	rm /etc/passwd
 	mv -v /data/etc/passwd /etc/passwd
 	rm /etc/group
@@ -27,4 +27,24 @@ all:
 	$(MAKE) -C postfix-3.5.4 CCARGS="-DUSE_TLS -I/usr/include/openssl/ -DUSE_SASL_AUTH -DUSE_CYRUS_SASL -I/usr/include/sasl -DHAS_LDAP" AUXLIBS="-lssl -lcrypto -lsasl2 -lldap -llber" makefiles
 	$(MAKE) -C postfix-3.5.4
 	cd postfix-3.5.4 && sh postfix-install -non-interactive daemon_directory=/usr/lib/postfix manpage_directory=/usr/share/man html_directory=/usr/share/doc/postfix-3.5.4/html readme_directory=/usr/share/doc/postfix-3.5.4/readme
+	@echo "$$POSTFIX_SERVICE" > /lib/systemd/system/postfix.service
+	systemctl enable postfix.service
 	rm -rf postfix-3.5.4
+
+postfix-service:
+define POSTFIX_SERVICE
+[Unit]
+Description=Postfix Mail Transport Agent
+After=network.target
+
+[Service]
+Type=forking
+PIDFile=/var/spool/postfix/pid/master.pid
+ExecStart=/usr/sbin/postfix start
+ExecStop=/usr/sbin/postfix stop
+ExecReload=/usr/sbin/postfix reload
+Restart=always
+
+[Install]
+WantedBy=multi-user.target
+export POSTFIX_SERVICE