Run `npm ci` instead of `npm install` when deploying

To ensure reproducible builds.
diff --git a/Dockerfile b/Dockerfile
index 3f42b6b..8a80046 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -3,7 +3,7 @@
 WORKDIR /app
 # Install dependencies based only on package*.json for better caching.
 COPY frontend/package*.json ./
-RUN npm install
+RUN npm ci
 COPY frontend ./
 RUN npm run build