Add retry logic for transient login failures

Adds configurable retry mechanism with basic exponential backoff to handle intermittent failures when authenticating to container registries, particularly GCP (GAR/GCR) where I'm seeing errors intermittently.

- Add retry-attempts input (default: 0 for backward compatibility, making it opt in)
- Add retry-delay input (default: 5000ms)
- Implement exponential backoff retry logic in docker login
  - Chose to just write a simple retry function vs. going with a library
- Retry all errors except 5xxs
  - I'm seeing intermittent 401 failures
- Add tests for retry behavior
- Update README with new input parameters

Signed-off-by: Naush Korai <naush.korai@mixpanel.com>

__tests__/docker.test.ts

@@ -2,6 +2,7 @@ import {expect, jest, test} from '@jest/globals';
 import * as path from 'path';
 
 import {loginStandard, logout} from '../src/docker';
+import {RetryArgs} from '../src/context';
 
 import {Docker} from '@docker/actions-toolkit/lib/docker/docker';
 
@@ -62,3 +63,103 @@ test('logout calls exec', async () => {
     ignoreReturnCode: true
   });
 });
+
+test('loginStandard retries on failure', async () => {
+  jest.useFakeTimers();
+  let attemptCount = 0;
+
+  // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+  // @ts-ignore
+  const execSpy = jest.spyOn(Docker, 'getExecOutput').mockImplementation(async () => {
+    attemptCount++;
+    if (attemptCount < 3) {
+      return {
+        exitCode: 1,
+        stdout: '',
+        stderr: 'Error: timeout exceeded'
+      };
+    }
+    return {
+      exitCode: 0,
+      stdout: 'Login Succeeded',
+      stderr: ''
+    };
+  });
+
+  const username = 'dbowie';
+  const password = 'groundcontrol';
+  const registry = 'https://ghcr.io';
+  const retryArgs: RetryArgs = {attempts: 3, delayMs: 100};
+
+  const loginPromise = loginStandard(registry, username, password, undefined, retryArgs);
+  await jest.runAllTimersAsync();
+  await loginPromise;
+
+  expect(execSpy).toHaveBeenCalledTimes(3);
+  expect(attemptCount).toBe(3);
+
+  jest.useRealTimers();
+});
+
+test('loginStandard does not retry when attempts is 0', async () => {
+  // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+  // @ts-ignore
+  const execSpy = jest.spyOn(Docker, 'getExecOutput').mockImplementation(async () => {
+    return {
+      exitCode: 1,
+      stdout: '',
+      stderr: 'Error: timeout exceeded'
+    };
+  });
+
+  const username = 'dbowie';
+  const password = 'groundcontrol';
+  const registry = 'https://ghcr.io';
+  const retryArgs: RetryArgs = {attempts: 0, delayMs: 100};
+
+  await expect(loginStandard(registry, username, password, undefined, retryArgs)).rejects.toThrow('timeout exceeded');
+
+  expect(execSpy).toHaveBeenCalledTimes(1);
+});
+
+test('loginStandard fails after max retries', async () => {
+  // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+  // @ts-ignore
+  const execSpy = jest.spyOn(Docker, 'getExecOutput').mockImplementation(async () => {
+    return {
+      exitCode: 1,
+      stdout: '',
+      stderr: 'Error: timeout exceeded'
+    };
+  });
+
+  const username = 'dbowie';
+  const password = 'groundcontrol';
+  const registry = 'https://ghcr.io';
+  const retryArgs: RetryArgs = {attempts: 2, delayMs: 10};
+
+  await expect(loginStandard(registry, username, password, undefined, retryArgs)).rejects.toThrow('timeout exceeded');
+
+  expect(execSpy).toHaveBeenCalledTimes(3);
+});
+
+test('loginStandard does not retry on 5xx errors', async () => {
+  // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+  // @ts-ignore
+  const execSpy = jest.spyOn(Docker, 'getExecOutput').mockImplementation(async () => {
+    return {
+      exitCode: 1,
+      stdout: '',
+      stderr: 'Error: 500 Internal Server Error'
+    };
+  });
+
+  const username = 'dbowie';
+  const password = 'groundcontrol';
+  const registry = 'https://ghcr.io';
+  const retryArgs: RetryArgs = {attempts: 3, delayMs: 100};
+
+  await expect(loginStandard(registry, username, password, undefined, retryArgs)).rejects.toThrow('500 Internal Server Error');
+
+  expect(execSpy).toHaveBeenCalledTimes(1);
+});