Check postfix configuration at build

This is handy to verify wether a given mail was sent encrypted.

postfix/Dockerfile

@@ -27,7 +27,7 @@ RUN echo '' > /etc/postfix/main.cf \
     && postconf -e recipient_delimiter='+' \
     && postconf -e smtpd_banner='$myhostname ESMTP $mail_name' \
     && postconf -e smtpd_use_tls='yes' \
-    && postconf -e smtpd_tls_loglevel='0' \
+    && postconf -e smtpd_tls_loglevel='1' \
     && postconf -e smtpd_tls_key_file='/tls/privkey' \
     && postconf -e smtpd_tls_cert_file='/tls/fullchain' \
     && postconf -e smtpd_tls_security_level='may' \
@@ -38,8 +38,11 @@ RUN echo '' > /etc/postfix/main.cf \
     && postconf -e smtpd_tls_exclude_ciphers='aNULL' \
     && postconf -e smtpd_tls_dh1024_param_file='/etc/postfix/dh-params/2048.pem' \
     && postconf -e smtpd_tls_eecdh_grade='strong' \
+    && postconf -e smtpd_tls_CApath='/etc/ssl/certs' \
+    && postconf -e smtp_tls_loglevel='1' \
     && postconf -e smtp_tls_security_level='may' \
     && postconf -e smtp_tls_mandatory_ciphers='medium' \
+    && postconf -e smtp_tls_CApath='/etc/ssl/certs' \
     && postconf -e tls_preempt_cipherlist='yes' \
     && postconf -e smtpd_sasl_auth_enable='yes' \
     && postconf -e smtpd_sasl_type='dovecot' \
@@ -52,6 +55,8 @@ RUN echo '' > /etc/postfix/main.cf \
     && postconf -e enable_long_queue_ids='yes' \
     && postconf -e strict_rfc821_envelopes='yes' \
     && postconf -e maillog_file='/dev/stdout' \
+    && touch /etc/postfix/virtual_mailbox_maps.cf \
+    && postfix check \
     && newaliases \
     && touch /etc/postfix/access_sender