Add test secret as arg

Dockerfile

@@ -8,6 +8,9 @@ ENV LIVEBOOK_DATA_PATH "/data"
 ENV LIVEBOOK_PORT 7860
 ENV LIVEBOOK_IFRAME_PORT 7861
 
+ARG LIVEBOOK_SECRET
+RUN echo $LIVEBOOK_SECRET > /secret.txt
+
 EXPOSE 7860
 EXPOSE 7861