create build-time random string for SECRET_KEY for prod

Dockerfile

@@ -20,6 +20,8 @@ RUN pip3 install --upgrade pillow --user
 EXPOSE 443
 EXPOSE 55432
 COPY . .
+RUN echo $RANDOM | md5sum | head -c 30 > /code/.sk
+RUN chmod 600 .sk
 RUN date > internal/build-date.txt
 RUN git log -n 15 > internal/git-log.txt
 RUN ln -s /code/TODO /code/internal/TODO