chore(docker): use supported Node 22 base image

[dvd233]

💻 变更类型 | Change Type

• feat
• fix
• refactor
• perf
• style
• test
• docs
• ci
• chore
• build

🔀 变更说明 | Description of Change

The published Docker image currently derives every stage, including the final runner, from node:18-alpine. Node.js 18 reached end of life on 2025-04-30 and no longer receives security fixes.

This change updates the shared base image to node:22-alpine, which keeps the existing deps / builder / runner stage layout and the current amd64/arm64 publishing workflow while moving the shipped runtime onto a supported LTS line.

Closes #6864.

📝 补充信息 | Additional Information

Validation:

• Node.js official release schedule: Node 18 EOL 2025-04-30; Node 22 EOL 2027-04-30.
• Docker Hub currently publishes an active node:22-alpine multi-architecture image including amd64 and arm64.
• tsc --noEmit passed.
• yarn build completed and produced the standalone Next.js output on this branch.
• Dockerfile stage assertion confirms the final runner still inherits the shared base.
• git diff --check passed.

A local Docker daemon was not available, so I did not run the image build itself; the existing Docker workflow will exercise the Linux amd64/arm64 build.