fix: install Docker CLI in CI/CD pipeline

- Add Docker CLI installation to frontend-build and backend-build jobs
- Gitea runner uses minimal image without Docker CLI pre-installed
- Docker daemon is available via DinD but CLI needs to be installed
- Fixes 'Unable to locate executable file: docker' error

This resolves the CI/CD build failures by ensuring Docker CLI is available
for the docker/build-push-action to work properly.

.gitea/workflows/ci.yml

@@ -42,6 +42,10 @@ jobs:
     if: gitea.event_name == 'push' && gitea.ref == 'refs/heads/master'
     steps:
       - uses: actions/checkout@v4
+      - name: Install Docker CLI
+        run: |
+          apt-get update
+          apt-get install -y docker.io
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
       - name: Log in to Harbor
@@ -95,6 +99,10 @@ jobs:
     if: gitea.event_name == 'push' && gitea.ref == 'refs/heads/master'
     steps:
       - uses: actions/checkout@v4
+      - name: Install Docker CLI
+        run: |
+          apt-get update
+          apt-get install -y docker.io
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
       - name: Log in to Harbor