feat: Add Docker Build and Push to CI Pipeline (#10)

## Implementation

- [x] Add docker build to CI pipeline
- [x] Publish docker image to GitHub

Each micro service is built and published separately. 

 - In branches other than `master`, only build will run. 
 - In the `master` branch, both build and publish will run. 

## Test
 - GitHub Actions for Docker Build when running at PR
<img width="200" height="400" alt="image"
src="https://github.com/user-attachments/assets/64e08088-f0c5-44a7-aa2a-438032c77377"
/>

<img width="300" height="204" alt="image"
src="https://github.com/user-attachments/assets/936b2ad1-2892-4813-9678-1a37dca0f7a9"
/>

Author: xGladiate

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

@@ -0,0 +1,86 @@
+name: Build & Push Docker images
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches: 
+      - master
+
+permissions:
+  packages: write
+
+jobs: 
+  docker:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        service:
+          # Add one entry per microservice
+          - name: matching-ui
+            context: ./ui-services/matching-ui-service
+          - name: questions-ui
+            context: ./ui-services/question-ui-service
+          - name: collab-ui
+            context: ./ui-services/collab-ui-service
+          - name: user-ui
+            context: ./ui-services/user-ui-service
+          - name: history-ui
+            context: ./ui-services/history-ui-service
+
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v4
+      
+      - name: Force owner to lowercase
+        id: lc
+        run: echo "value=${GITHUB_REPOSITORY_OWNER,,}" >> "$GITHUB_OUTPUT"
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        if: ${{ github.event_name != 'pull_request' }}
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ steps.lc.outputs.value }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract Docker metadata (tags/labels)
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: |
+            ghcr.io/${{ steps.lc.outputs.value }}/peerprep-${{ matrix.service.name }}
+          tags: |
+            type=raw,value=${{ github.sha }}
+            type=ref,event=branch
+            type=ref,event=tag
+            type=raw,value=latest,enable=${{ github.ref_name == 'master' }}
+
+      - name: Build (no push for PRs)
+        if: ${{ github.event_name == 'pull_request' }}
+        uses: docker/build-push-action@v6
+        with:
+          context: ${{ matrix.service.context }}
+          file: ${{ matrix.service.dockerfile }}
+          push: false
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+      - name: Build & Push
+        if: ${{ github.event_name != 'pull_request' }}
+        uses: docker/build-push-action@v6
+        with:
+          context: ${{ matrix.service.context }}
+          file: ${{ matrix.service.dockerfile }}
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+