Add build step to CI

.github/workflows/build.yml

@@ -0,0 +1,54 @@
+name: Build
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+concurrency:
+  group: build-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    name: Next.js build
+    runs-on: ubuntu-latest
+
+    # Placeholder values only. `next build` needs these to be present and
+    # well-formed because a few modules construct clients at import time, but
+    # nothing here is contacted during the build.
+    env:
+      DATABASE_URL: postgresql://user:password@localhost:5432/db
+      DIRECT_URL: postgresql://user:password@localhost:5432/db
+      NEXT_PUBLIC_SUPABASE_URL: https://placeholder.supabase.co
+      NEXT_PUBLIC_SUPABASE_PUBLISHABLE_KEY: placeholder-key
+      NEXT_PUBLIC_BASE_URL: http://localhost:3000
+      NEXT_PUBLIC_REDIRECT_URL: http://localhost:3000/auth/callback
+      NEXT_PUBLIC_COLONY_URL: http://localhost:3000
+      MAILGUN_DOMAIN: mg.example.com
+      MAILGUN_API_KEY: placeholder-key
+      NEWSLETTER_URI: http://localhost:3000/api/newsletter
+      NOTION_DB_ID: placeholder
+      NOTION_DB_SECRET: placeholder
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up pnpm
+        uses: pnpm/action-setup@v4.0.0
+        with:
+          version: 10.14.0
+
+      - name: Set up Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: pnpm
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Build
+        run: pnpm build