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/build.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+if [ "$1" == "" ] ; then
+    printf "\nNisi zadao verziju Docker image-a koji treba buildati"
+    printf "\n\nSintaksa:\n\n    build-prod-img.sh 1.0.0\n\n"
+    exit 1
+fi
+
+printf "\nBUILD START ...\n\n"
+
+IMAGE_VERSION=$1
+
+docker build . -t utility-bills-tracker:$IMAGE_VERSION
+
+printf "\nBUILD DONE!\n\n"