Remove 'unstable' tag from Docker image in workflow configuration

Author: tacxou

.github/workflows/docker-image.yml

@@ -37,7 +37,7 @@ jobs:
         id: meta
         uses: docker/metadata-action@9ec57ed1fcdbf14dcef7dfbe97b2010124a938b7
         with:
-          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:unstable
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
 
       - name: Build and push Docker image
         uses: docker/build-push-action@f2a1d5e99d037542a71f64918e516c093c6f3fc4