refactor: move Docker build files to web-app directory

Moved Dockerfile, build.sh, and .dockerignore to web-app/ for better
project encapsulation. Each project now contains its own build configuration.

## Changes
- Moved Dockerfile to web-app/ and simplified paths
- Moved build.sh to web-app/
- Moved .dockerignore to web-app/
- Updated Dockerfile to work from web-app/ context (no workspace references)
- Updated documentation for new build workflow

## Build Workflow
- Build: Run from web-app/ directory (`cd web-app && ./build.sh 2.20.0`)
- Deploy: Run from repository root (`./deploy-standalone.sh 2.20.0`)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

web-app/.dockerignore

@@ -0,0 +1,7 @@
+.dockerignore
+.git
+.vscode
+.gitignore
+node_modules
+mongo
+mongo-volume