Consider supporting `shell` input

Please consider adding a simple `shell` input that matches the behavior of the `shell` argument for built-in [`steps`](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idstepsshell).

Because `devcontainers/ci` will be used alongside other "native" execution environments which use `shell: bash`, adding this flag can improve consistency and readability in multi-environment action scripts.