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

Dockerfile

@@ -2,6 +2,13 @@ FROM debian:stable-slim
 
 ENV DEBIAN_FRONTEND=noninteractive
 
+# Default is to run cron job in the 10th minute of each hour
+# You can override at runtime with -e CRON_MINUTE=xx
+ENV CRON_MINUTE=10
+# Default port for http server inside container.
+# You can override at runtime with -e PORT=xxxx
+ENV PORT=8080
+
 # Install required packages: curl, cron, busybox (httpd), ca-certificates
 RUN apt-get update \
 	&& apt-get install -y --no-install-recommends curl cron busybox ca-certificates gnupg dirmngr \
@@ -25,14 +32,8 @@ COPY start.sh /usr/local/bin/start.sh
 # Ensure scripts are executable
 RUN chmod +x /usr/local/bin/run_speed_test.sh /usr/local/bin/start.sh
 
-# Add cron job to run every hour at minute 0
-RUN printf "0 * * * * root /usr/local/bin/run_speed_test.sh >> /var/log/cron.log 2>&1\n" > /etc/cron.d/speedtest-cron \
-	&& chmod 0644 /etc/cron.d/speedtest-cron
-
 # Expose default HTTP port and declare data volume
 VOLUME ["/var/www/data/speed"]
-# Default port for http server inside container. You can override at runtime with -e PORT=xxxx
-ENV PORT=8080
 EXPOSE 8080
 
 # Start script will launch cron and the HTTP server