fix: update CI environment variable reference

Checking to see if an incorrect syntax was used to reference the `image_tags` environnment variable which was added in the previous step.

.github/workflows/devcontainer-build-and-push.yaml

@@ -40,6 +40,6 @@ jobs:
         with:
           subFolder: .github
           imageName: ghcr.io/${{ github.repository }}
-          imageTag: ${{ image_tags }}
+          imageTag: $image_tags
           cacheFrom: ghcr.io/${{ github.repository }}
           push: always