docs+runtime: warn against mounting volumes under $GITHUB_WORKSPACE

[mariuz]

Mounting a volume host-path under ${{ github.workspace }} breaks silently: actions/checkout (default clean: true) runs after this action in the typical workflow order and wipes the workspace, destroying the mount point while the Firebird container keeps running against a now-missing directory. This was the root cause of mapped-filesystem test failures observed in the Jaybird CI.

Changes

• README.md — added a [!WARNING] callout under the volumes parameter:

  • Explains the checkout-order hazard

  • Recommends ${{ runner.temp }} (job-scoped, never touched by checkout) as the correct host path

  • Includes a ready-to-use workflow snippet:

    - uses: juarezr/firebirdsql-github-action@v2
      with:
        volumes: '${{ runner.temp }}/firebird-data:/var/lib/firebird/data'
    - uses: actions/checkout@v4
    - name: Run tests
      run: ./gradlew test -Ptest.db.mapped=${{ runner.temp }}/firebird-data

• entrypoint.sh — non-fatal runtime warning when a volume host-path begins with $GITHUB_WORKSPACE/ (slash-bounded to avoid false-positives on sibling directories):

  ## WARNING: Volume host path '...' is inside $GITHUB_WORKSPACE.
  ## WARNING: If actions/checkout runs after this step (default), it will delete
  ## WARNING: the workspace directory (clean: true), breaking the volume mount.
  ## WARNING: Use ${{ runner.temp }} instead to avoid this problem.
  

[mrotteveel]

I tried the change as suggested here, but it didn't work for me.