ci: add GitHub Actions workflow for Docker image build and push

Add automated workflow to build and publish Docker images to GHCR:
- Builds on push to master/main branches
- Builds on version tags (v*)
- Builds on pull requests (without pushing)
- Supports manual workflow dispatch

Features:
- Multi-platform support (linux/amd64)
- Automatic semantic versioning from git tags
- GitHub Actions cache for faster builds
- Build attestation for supply chain security
- Images pushed to ghcr.io/netresearch/timetracker-ui

Tags generated:
- Branch name (e.g., master, main)
- PR number (e.g., pr-123)
- Semantic versions (e.g., v1.0.0, v1.0, v1)
- Git SHA (e.g., master-abc1234)
- 'latest' for default branch

Author: CybotTM

.github/workflows/docker-build-push.yml

@@ -0,0 +1,73 @@
+name: Build and Push Docker Image
+
+on:
+  push:
+    branches: [ master, main ]
+    tags:
+      - 'v*'
+  pull_request:
+    branches: [ master, main ]
+  workflow_dispatch:
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+      attestations: write
+      id-token: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v5
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GitHub Container Registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=sha,prefix={{branch}}-
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push Docker image
+        id: push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          platforms: linux/amd64
+
+      - name: Generate artifact attestation
+        if: github.event_name != 'pull_request'
+        uses: actions/attest-build-provenance@v2
+        with:
+          subject-name: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          subject-digest: ${{ steps.push.outputs.digest }}
+          push-to-registry: true
+