relayhost settings, add ca-certificates package

Dockerfile

@@ -38,6 +38,15 @@ ENV POSTSCREEN_DNSBL_ACTION="enforce"
 ENV POSTSCREEN_DNSBL_THRESHOLD="3"
 ENV POSTSCREEN_DNSBL_WHITELIST_THRESHOLD="-1"
 ENV POSTSCREEN_GREET_ACTION="enforce"
+ENV RELAYHOST=""
+ENV SMTP_SASL_AUTH_ENABLE=no
+ENV SMTP_SASL_PASSWORD_MAPS=""
+ENV SMTP_SASL_SECURITY_OPTIONS="" # default noplaintext,noanonymous
+ENV SMTP_TLS_CAFILE=""
+ENV SMTP_TLS_MANDATORY_PROTOCOLS="" # default !SSLv2,!SSLv3
+ENV SMTP_TLS_NOTE_STARTTLS_OFFER=no
+ENV SMTP_TLS_SECURITY_LEVEL=""
+ENV SMTP_TLS_SESSION_CACHE_DATABASE=""
 
 # TODO: template access lists
 
@@ -50,6 +59,7 @@ RUN echo "_dev_null: /dev/null" > /etc/aliases \
 	postfix-sqlite \
 	libsasl2-modules \
 	ssl-cert \
+	ca-certificates \
   && rm -rf /var/lib/apt/lists/* \
   && cp /usr/share/postfix/main.cf.debian /etc/postfix/main.cf \
   && mkdir -p /etc/postfix/rules \