Add SMTP Config to your .env file:

``` 
# COMPOSE_FILE="$COMPOSE_FILE:compose.smtp.yml"
# See https://github.com/nextcloud/docker#auto-configuration-via-environment-variables for default values
# SMTP_AUTHTYPE=
# SMTP_HOST=
# SMTP_SECURE=
# SMTP_NAME=
# SMTP_PORT=
# MAIL_FROM_ADDRESS=
# MAIL_DOMAIN=
# SECRET_SMTP_PASSWORD_VERSION=v1
```