chore: trim trailing whitespace WebApp.Dockerfile

[tienthanh2509]

Minor stuff Trim trailing whitespace WebApp.Dockerfile

• [x] I have built and tested the code locally and in a deployed app
• [x] This is a change for all users of this app. No code or asset is specific to my use case or my organization.
• [x] I didn't break any existing functionality :smile:

[tienthanh2509]

@microsoft-github-policy-service agree