feat(docker): disable Next.js telemetry in containerized deployments

- Add NEXT_TELEMETRY_DISABLED=1 to Dockerfile (builder and runner stages)
- Add NEXT_TELEMETRY_DISABLED=1 to docker-compose.yml
- Add build-args to GitHub Actions workflow
- Document environment variables in README

This ensures no telemetry data is collected from containerized
deployments, respecting user privacy and reducing network overhead.

.github/workflows/docker-build.yml

@@ -63,6 +63,8 @@ jobs:
           cache-from: type=gha
           cache-to: type=gha,mode=max
           platforms: linux/amd64,linux/arm64
+          build-args: |
+            NEXT_TELEMETRY_DISABLED=1
 
       - name: Image digest
         run: echo ${{ steps.meta.outputs.digest }}