[feature request] support relayhost

[eleith]

Currently

postfix relayhost and a user/pass need to be manually added to enable support of using an external smtp relay for outgoing emails (useful, since port 25 is blocked on some cloud providers)

Request

instead of manually adding, these could be configured by environmental variables

for example, add the following environmental variables

POSTFIX_RELAYHOST
POSTFIX_RELAYUSER
POSTFIX_RELAYPASSWORD

if those are set, then in the postfix shell scripts

cat the following into /etc/postfix/main.cf

relayhost=${POSTFIX_RELAYHOST}

then cat the following into /etc/postfix/sasl_password

${POSTFIX_RELAYHOST} ${POSTFIX_RELAYUSER}:${POSTFIX_RELAYPASSWORD}

then run the following command

postmap /etc/postfix/sasl_passwd