Support `docker-cmd` option for renovate-config-validator

Background

We use this action for renovate-config-validator as well.

    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
      - run: |
          echo "$script" > renovate-config-validator.sh
          chmod +x renovate-config-validator.sh
        env:
          script: |
            #!/bin/bash
            set -eux -o pipefail
            exec renovate-config-validator /workspace/renovate.*

      - uses: renovatebot/github-action@a889a8abcb11ef7feaafaf5e483ea01d4bf7774e # v43.0.5
        with:
          token: ${{ secrets.GITHUB_TOKEN }}
          docker-cmd-file: renovate-config-validator.sh
          docker-volumes: ${{ github.workspace }}:/workspace

Feature request

It would be nice if it can pass the docker command directly.

For example, if this action supports docker-cmd, the workflow would be like:

      - uses: renovatebot/github-action@a889a8abcb11ef7feaafaf5e483ea01d4bf7774e # v43.0.5
        with:
          token: ${{ secrets.GITHUB_TOKEN }}
          docker-cmd: renovate-config-validator /workspace/renovate.*
          docker-volumes: ${{ github.workspace }}:/workspace

Start by tracing how action inputs invoke the Docker command and how the existing docker-cmd-file option is handled. Add support for passing the requested docker-cmd value directly while preserving docker-volumes; done means the example workflow can run renovate-config-validator without creating a shell script.