feat(caddy): upload binary to S3

- Upload the compiled binary to any S3-compatible storage via AWS CLI
- Read endpoint, region and bucket from repo variables (S3_ENDPOINT, S3_REGION, S3_BUCKET)
- Skip the upload when S3_BUCKET is unset
- Document the configuration in README

.github/workflows/build-caddy.yaml

@@ -40,4 +40,16 @@ jobs:
         uses: actions/upload-artifact@v4
         with:
           name: caddy-custom-linux-amd64
-          path: caddy
+          path: caddy
+
+      - name: Upload binary to S3
+        if: ${{ vars.S3_BUCKET != '' }}
+        env:
+          AWS_ACCESS_KEY_ID: ${{ secrets.S3_ACCESS_KEY }}
+          AWS_SECRET_ACCESS_KEY: ${{ secrets.S3_SECRET_KEY }}
+          AWS_DEFAULT_REGION: ${{ vars.S3_REGION || 'cn-hongkong' }}
+        run: |
+          aws configure set default.s3.addressing_style virtual
+          aws s3 cp caddy "s3://${{ vars.S3_BUCKET }}/caddy-${{ inputs.caddy_version }}-linux-amd64" \
+            --endpoint-url "${{ vars.S3_ENDPOINT }}" \
+            --no-progress