fix: Rename Docker ignore file to standard .dockerignore

[Prasanjeet-Microsoft]

Purpose

• This PR renames WebApp.dockerignore to .dockerignore to ensure it is recognized by the Docker CLI during image build.
• The .dockerignore file prevents unnecessary files and directories (e.g., node_modules, .venv, etc.) from being sent to the Docker daemon as part of the build context, which improves build performance and keeps image sizes smaller.

Does this introduce a breaking change?

• Yes
• No

Golden Path Validation

• I have tested the primary workflows (the "golden path") to ensure they function correctly without errors.

Deployment Validation

• I have validated the deployment process successfully and all services are running as expected with this change.

What to Check

Verify that the following are valid

• I have built and tested the code locally and in a deployed app
• For frontend changes, I have pulled the latest code from main, built the frontend, and committed all static files.
• This is a change for all users of this app. No code or asset is specific to my use case or my organization.

Other Information

• Verify that the .dockerignore file is correctly recognized during the Docker build and the intended files are excluded