Add name output to identify builder instance name

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

setup-buildx/src/buildx.ts

@@ -19,6 +19,15 @@ export async function isAvailable(): Promise<Boolean> {
   });
 }
 
+export async function countBuilders(): Promise<number> {
+  return await exec.exec(`docker`, ['buildx', 'ls'], true).then(res => {
+    if (res.stderr != '' && !res.success) {
+      throw new Error(`Cannot list builders: ${res.stderr}`);
+    }
+    return (res.stdout.trim().split(`\n`).length - 1) / 2;
+  });
+}
+
 export async function install(inputVersion: string, dockerConfigHome: string): Promise<string> {
   const release: github.GitHubRelease | null = await github.getRelease(inputVersion);
   if (!release) {