ci: build docker image before run

.gitea/workflows/build.yml

@@ -4,6 +4,9 @@ on:
   push:
     branches:
       - main
+  schedule:
+    # 00:00 on the 1st of every month
+    - cron: '0 0 1 * *'
   workflow_dispatch:
 
 jobs:
@@ -19,7 +22,9 @@ jobs:
 
       - name: Build Linux binary (docker)
         working-directory: .build/prod
-        run: docker compose run --rm prod
+        run: |
+          docker compose build
+          docker compose run --rm prod
 
       - name: Create Gitea release & upload assets
         env: