Allow building buildx from source

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

__tests__/git.test.ts

@@ -0,0 +1,9 @@
+import * as git from '../src/git';
+
+describe('git', () => {
+  it('returns git remote ref', async () => {
+    const ref: string = await git.getRemoteSha('https://github.com/docker/buildx.git', 'refs/pull/648/head');
+    console.log(`ref: ${ref}`);
+    expect(ref).toEqual('f11797113e5a9b86bd976329c5dbb8a8bfdfadfa');
+  });
+});