Option to disable SMTP creds

[cron410]

I have an old camera system and copier that are the only devices using this mail relay. They do not have an option for SMTP passwords. I was able to do a dirty hack and edit your entrypoint.sh to remove the following code:

[ -z "$SMTP_LOGIN" -o -z "$SMTP_PASSWORD" ] && {
	echo "SMTP_LOGIN and SMTP_PASSWORD _must_ be defined" >&2
	exit 1
}