Update build configuration to use stable tag and upstream image

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: [dev, amd64]
     env:
       IMAGE: gitea.ceperka.net/cx/core
-      TAG: "43"
+      TAG: "stable"
     steps:
       - uses: actions/checkout@v4
 
@@ -20,7 +20,7 @@ jobs:
         run: echo "${{ secrets.CICD_PASSWORD }}" | docker login gitea.ceperka.net -u ${{ secrets.CICD_USER }} --password-stdin
 
       - name: Download latest upstream image
-        run: docker pull quay.io/fedora/fedora-coreos:testing
+        run: docker pull quay.io/fedora/fedora-coreos:stable
 
       - name: Build
         run: docker build -t $IMAGE:$TAG .