ci: use npm install instead of npm ci to avoid lock file version mismatch

Lock file was generated with npm 11 (Node 24), CI runs npm 10 (Node 22).
npm install avoids the strict sync check and matches the Dockerfile.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitea/workflows/docker-build.yml

@@ -29,7 +29,7 @@ jobs:
           cache: 'npm'
 
       - name: Install dependencies
-        run: npm ci
+        run: npm install
 
       - name: Run tests
         run: npm test