Fix docker-image.yml: indentation, Dockerfile path, and image tag

[Copilot]

The workflow file had invalid YAML indentation, referenced a non-existent root-level Dockerfile, and used an untraceable timestamp-based image tag.

Changes

• Indentation — fixed steps entries to be properly nested under steps:
• Dockerfile path — Dockerfile → .devcontainer/Dockerfile (the actual Dockerfile in this repo)
• Image tag — $(date +%s) → ${{ github.sha }} for commit-traceable image tags

# Before
    steps:
    - uses: actions/checkout@v4
    - name: Build the Docker image
      run: docker build . --file Dockerfile --tag my-image-name:$(date +%s)

# After
    steps:
      - uses: actions/checkout@v4
      - name: Build the Docker image
        run: docker build . --file .devcontainer/Dockerfile --tag my-image-name:${{ github.sha }}