fixup! ci(.gitlab-ci): use a Docker image for Decker builds

.gitlab-ci.yml

@@ -37,6 +37,8 @@ build:
     echo "Generate Decks with Decker."
     /usr/local/bin/decker decks
     echo "Successfully built Decker slides."
+    ls -a
+    ls -a public/
   artifacts:
     paths:
       - $BUILD_OUTPUT