after installing with ansible, change the docker-compose.yml file on you server passing the api in the ENV variable:

the postfix section now looks like this

  postfix:
    image: mwader/postfix-relay
    environment:
      - POSTFIX_myhostname=[yourdomain.tld]
      - POSTFIX_inet_protocols=ipv4
      - POSTFIX_smtp_sasl_auth_enable=yes
      - POSTFIX_smtp_sasl_password_maps=static:apikey:[api-key]
      - POSTFIX_smtp_tls_security_level=may
      - POSTFIX_smtp_sasl_security_options=noanonymous
      - POSTFIX_relayhost=smtp.sendgrid.net:587
      - OPENDKIM_DOMAINS=[yourdomain.tld]
    restart: "always"