fix(ci): add auth header to Docker MCP endpoint test (czlonkowski#726)

The /mcp endpoint requires authentication since v2.47.6 (GHSA-75hx-xj24-mqrw),
but the Docker image smoke test was still calling it without a Bearer token,
causing a 401 and failing the build.

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: czlonkowski

.github/workflows/docker-build-n8n.yml

@@ -148,8 +148,8 @@ jobs:
           # Test health endpoint
           curl -f http://localhost:3000/health || exit 1
           
-          # Test MCP endpoint
-          curl -f http://localhost:3000/mcp || exit 1
+          # Test MCP endpoint (requires auth since v2.47.6)
+          curl -f -H "Authorization: Bearer test-token-minimum-32-chars-long" http://localhost:3000/mcp || exit 1
           
           # Cleanup
           docker stop n8n-mcp-test