Fix: Update Variable Names and Syntax in Backend Container

[kajgnis]

Adjusted the backend container code to ensure compatibility with the previous tutorial.

Description

This pull request addresses several issues in the backend container code:

1. Changed image=backend.base_image_name to image=backend.repo_digest to resolve the AttributeError: 'RemoteImage' object has no attribute 'base_image_name' error.

2. Updated Output.concat to pulumi.Output.concat to resolve the NameError: name 'Output' is not defined error.

3. Modified config.require_secret("mongo_password") to config.require_secret("mongoPassword") to resolve the Missing required configuration variable 'pulumi_first_project:mongo_password' error.

Checklist:

• [n/a] I have reviewed the style guide.
• [n/a] If blogging, I have reviewed the blogging guide.
• [n/a] I have manually confirmed that all new links work.
• [n/a] I added aliases (i.e., redirects) for all filename changes.
• [n/a] If making css changes, I rebuilt the bundle.