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>

action.yml

@@ -29,6 +29,14 @@ inputs:
   registry-auth:
     description: 'Raw authentication to registries, defined as YAML objects'
     required: false
+  retry-attempts:
+    description: 'Number of retry attempts for transient failures'
+    default: '0'
+    required: false
+  retry-delay:
+    description: 'Delay between retries in milliseconds'
+    default: '5000'
+    required: false
 
 runs:
   using: 'node20'