Improve docker setup and update scripts

hosting/Dockerfile

@@ -1,15 +1,18 @@
-# Use lightweight Nginx + Git
+# environment variables necessary to run:
+# REPO_URL  the url of this repo
+# BRANCH    the branch to work at
+
 FROM nginx:alpine
 
-# Install git and bash
 RUN apk add --no-cache git bash
-
-# Copy nginx config
 COPY nginx.conf /etc/nginx/nginx.conf
 
-# Copy update script
 COPY update.sh /update.sh
 RUN chmod +x /update.sh
 
-# Start update loop + nginx
-CMD ["/bin/bash", "-c", "/update.sh & nginx -g 'daemon off;'"]
+# Add the cron job to run every 30 minutes
+# Redirecting to /proc/1/fd/1 ensures the script's echo statements show up in `docker logs`
+RUN echo "*/30 * * * * bash /update.sh > /proc/1/fd/1 2>&1" > /etc/crontabs/root
+
+# Start the cron daemon in the background (-b) and nginx in the foreground
+CMD crond -b && nginx -g 'daemon off;'