Added new parameter CRON_MINUTE to docker image

This can be used to start the cron on a specific minute within
the hour. By default, the cron will run in the 10th minute

start.sh

@@ -2,6 +2,9 @@
 
 set -e
 
+CRON_MINUTE=${CRON_MINUTE:-10}
+PORT=${PORT:-8080}
+
 # Ensure data directory exists
 mkdir -p /var/www/data/speed
 
@@ -17,20 +20,17 @@ chmod -R a+rwX /var/www/data || true
 # We run it in background so server starts promptly. The cron will run hourly.
 /usr/local/bin/run_speed_test.sh || true &
 
-# Ensure the cron job exists in root's crontab (so `crontab -l` shows it)
-CRON_ENTRY="* * * * * /usr/local/bin/run_speed_test.sh >> /var/log/cron.log 2>&1"
-if crontab -l 2>/dev/null | grep -F "$CRON_ENTRY" >/dev/null 2>&1; then
-	echo "cron entry already present"
+# Ensure the cron job exists in Cron
+CRON_FILE=/etc/cron.d/speedtest-cron
+if grep run_speed_test ${CRON_FILE}; then
+    echo "cron entry already present"
 else
-	(crontab -l 2>/dev/null; echo "$CRON_ENTRY") | crontab -
-	echo "installed cron entry into root crontab"
+    echo ${CRON_MINUTE} '* * * * /usr/local/bin/run_speed_test.sh >> /var/log/cron.log 2>&1' > ${CRON_FILE}
 fi
 
 # Start Debian cron in background
 service cron start || cron || true
 
-PORT=${PORT:-8080}
-
 # Start busybox httpd serving /var/www on configured port in foreground
 if command -v busybox >/dev/null 2>&1; then
     echo "starting busybox httpd on port ${PORT} serving /var/www"