I want to add backend functionality to send the survey responses as an email upon submission using Option 1 (a minimalist Go/Golang microservice).

Please complete the following tasks:

1. Go Microservice (`main.go`):
   - Create a lightweight HTTP server using standard Go library packages (`net/http`, `net/smtp`, `encoding/json`).
   - Create an endpoint (e.g., `POST /api/submit`) to accept JSON payload survey data from the front end.
   - Configure SMTP credentials and server settings via environment variables (e.g., `SMTP_HOST`, `SMTP_PORT`, `SMTP_USER`, `SMTP_PASS`, `TO_EMAIL`) via an .env file
   - Format the incoming JSON survey data into a clean, readable text body and dispatch the email upon request.
   - Handle CORS preflight (`OPTIONS`) and headers so the HTML/JS front end can POST to it smoothly.

2. Dockerization (`Dockerfile`):
   - Provide a multi-stage `Dockerfile` (e.g., compile with `golang:alpine`, copy to `scratch` or `alpine`) to ensure the resulting container image is ultra-lightweight (<20MB).

3. Front-End Integration:
   - Show how to update the existing JavaScript submission handler (using `fetch()`) to send the survey data as JSON to the new Go backend endpoint.

Please keep the code minimalist, robust, and well-commented.

when everything works, please create a simple docker-compose file that builds the project and runs the container. include traefik labels for reverse proxy routing.