fix: pin Redis service container image to 8.6

[fcostaoliveira]

Updates all Redis image references to redis:8.6 per org standard (8.6 minimum).

Changes:

• .github/workflows/test-basic-functionality-redis-vector-sets.yml: redis:8.2-rc1-bookworm → redis:8.6
• .github/workflows/test-basic-functionality-redisearch.yml: redis:8.2-rc1-bookworm → redis:8.6
• .github/workflows/docker-build-pr.yml: redis:8.2-rc1-alpine3.22 → redis:8.6

[github-actions]

🐳 Docker Build Validation

✅ Docker build successful!

Platforms tested:

• ✅ linux/amd64 (built and tested)
• ✅ linux/arm64 (build validated)

Git SHA: af803d92e6fdf05cb56ba615b67ac00ebe9ef5ae

Docker Hub Status: ✅ Docker Hub credentials configured

Image details:

• Single platform: vector-db-benchmark-pr:pr-55
• Multi-platform: vector-db-benchmark-pr:pr-55-multiplatform

Tests performed:

• ✅ Docker Hub credentials check
• ✅ Help command execution
• ✅ Python environment validation
• ✅ Redis connectivity test
• ✅ Benchmark execution test (redis-m-16-ef-64)
• ✅ Multi-platform build validation

The Docker image is ready for deployment! 🚀