Updated docker File

docker/Dockerfile

@@ -21,11 +21,12 @@ ADD https://api.github.com/repos/chrisb86/nsupdate/git/refs/heads/main /.git-has
 COPY nsupdate.sh /usr/local/bin/nsupdate.sh
 RUN chmod +x /usr/local/bin/nsupdate.sh
 
-## Setup cron job
-RUN echo "${SCHEDULE} sh /usr/local/bin/nsupdate.sh" >> /etc/crontabs/root
+COPY docker/entrypoint.sh /usr/local/bin/entrypoint.sh
+RUN chmod +x /usr/local/bin/entrypoint.sh
 
 ## Start crond
-CMD [ "crond", "-l", "2", "-f" ]
+ENTRYPOINT [ "/usr/local/bin/entrypoint.sh" ]
+CMD [ "crond", "-f" ]
 
 VOLUME /config
 VOLUME /log

docker/entrypoint.sh

@@ -1,4 +1,20 @@
-#! /bin/sh
+#!/bin/sh
 
-echo "${SCHEDULE} sh nsupdate.sh" >> /etc/crontabs/root
-crond -l 2 -f > /dev/stdout 2> /dev/stderr &
+set -eu
+
+SCHEDULE="${SCHEDULE:-* * * * *}"
+CRON_LOG_LEVEL="${CRON_LOG_LEVEL:-2}"
+CRON_FILE="/etc/crontabs/root"
+
+printf '%s /usr/local/bin/nsupdate.sh\n' "${SCHEDULE}" > "${CRON_FILE}"
+
+if [ "$#" -gt 0 ]; then
+	if [ "$1" = "crond" ]; then
+		shift
+		exec crond -l "${CRON_LOG_LEVEL}" "$@"
+	fi
+
+	exec "$@"
+fi
+
+exec crond -l "${CRON_LOG_LEVEL}" -f