build: use build_date argument for docker image instead of guessing from pacman root

.github/workflows/docker.yml

@@ -26,6 +26,10 @@ jobs:
 
       - uses: docker/setup-buildx-action@v3
 
+      - name: Set image date
+        id: args
+        run: echo "::set-output name=date::$(date -d yesterday +'%Y-%m-%d')"
+
       - name: Login to docker hub
         uses: docker/login-action@v3
         with:
@@ -53,6 +57,8 @@ jobs:
       - name: Build an image and push
         uses: docker/build-push-action@v6
         with:
+          build-args: |
+            BUILD_DATE=${{ steps.args.outputs.date }}
           file: docker/Dockerfile
           push: true
           tags: ${{ steps.meta.outputs.tags }}