support standalone mode and display version

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

__tests__/docker.test.ts

@@ -0,0 +1,16 @@
+import {describe, expect, it, jest} from '@jest/globals';
+import * as docker from '../src/docker';
+import * as exec from '@actions/exec';
+
+describe('isAvailable', () => {
+  it('cli', () => {
+    const execSpy = jest.spyOn(exec, 'getExecOutput');
+    docker.isAvailable();
+
+    // eslint-disable-next-line jest/no-standalone-expect
+    expect(execSpy).toHaveBeenCalledWith(`docker`, undefined, {
+      silent: true,
+      ignoreReturnCode: true
+    });
+  });
+});