feat: add Docker support and CI workflow

- Created a Dockerfile to build and deploy the application using a multi-stage build process.
- Added a GitHub Actions workflow to automatically build and publish the Docker image to GitHub Container Registry on pushes to the main branch.
- Updated go.mod and go.sum to reflect dependency upgrades and changes.
- Modified the RSS generator to extend the photo expiration time from 10 minutes to 12 hours.
- Updated rate limiter configuration to allow for environment variable customization.

internal/server/ratelimiter.go

@@ -40,7 +40,7 @@ func (r *RateLimiter) getLimiter(ip string, kind string) *rate.Limiter {
 	}
 
 	feedLim := rate.NewLimiter(rate.Every(3*time.Second), 5)
-	imageLim := rate.NewLimiter(rate.Every(5*time.Second), 4)
+	imageLim := rate.NewLimiter(rate.Every(5*time.Second), 4) // TODO: get from env
 
 	r.clients[ip] = &ClientLimiter{
 		feedLimiter:  feedLim,