Allow building buildx from source

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

__tests__/util.test.ts

@@ -0,0 +1,11 @@
+import * as util from '../src/util';
+
+describe('isValidUrl', () => {
+  test.each([
+    ['https://github.com/docker/buildx.git', true],
+    ['https://github.com/docker/buildx.git#refs/pull/648/head', true],
+    ['v0.4.1', false]
+  ])('given %p', async (url, expected) => {
+    expect(util.isValidUrl(url)).toEqual(expected);
+  });
+});