✨(build): make AWS credentials optional for registry cache

[Rhodri-Morgan]

Summary

Make AWS credentials optional for the build action. Registry-based layer caching is now an opt-in feature that only activates when all three AWS credentials are provided.

Changes

• Made AWS inputs optional: aws-region, account-id, and role-arn are now optional with empty string defaults
• Removed push-cache input: No longer needed since registry caching is always enabled when AWS credentials are present
• Split build step into two paths:
  • "Build image (with registry cache)" - runs when all AWS inputs are provided, writes cache back to ECR
  • "Build image (no cache)" - runs when any AWS input is missing, uses only local Docker cache
• Updated action description: Now states "optional registry-based layer caching"
• Updated README: Clarified that registry caching is optional and removed push-cache from example workflow

Testing

• Verified both code paths in the action
• Checked that conditional logic correctly gates AWS setup and ECR operations
• Confirmed documentation examples are updated

Checklist

• Code follows style guidelines
• Changes align with conventional commit format
• Documentation updated