How to user cron inside docker container - docker

Question

I tryed to add crontab inside docker image "jenkinsci/blueocean" but after it, jenkins does not start. Where could be the problem?
Many thanks in advance for any help.
<Dockerfile>
FROM jenkinsci/blueocean:1.17.0
USER root
ENV SUPERCRONIC_URL=https://github.com/aptible/supercronic/releases/download/v0.1.9/supercronic-linux-amd64 \
SUPERCRONIC=supercronic-linux-amd64 \
SUPERCRONIC_SHA1SUM=5ddf8ea26b56d4a7ff6faecdd8966610d5cb9d85
RUN curl -fsSLO "$SUPERCRONIC_URL" \
&& echo "${SUPERCRONIC_SHA1SUM} ${SUPERCRONIC}" | sha1sum -c - \
&& chmod +x "$SUPERCRONIC" \
&& mv "$SUPERCRONIC" "/usr/local/bin/${SUPERCRONIC}" \
&& ln -s "/usr/local/bin/${SUPERCRONIC}" /usr/local/bin/supercronic
ADD crontab /etc/crontab
CMD ["supercronic", "/etc/crontab"]
<crontab>
# Run every minute
*/1 * * * * echo "hello world"
commands:
$docker build -t jenkins_test .
$docker run -it -p 8080:8080 --name=container_jenkins jenkins_test

Answer 1

If use docker inspect jenkinsci/blueocean:1.17.0 you will it's entrypoint is:
"Entrypoint": [
"/sbin/tini",
"--",
"/usr/local/bin/jenkins.sh"
],
So, when start the container it will first execute next script.
/usr/local/bin/jenkins.sh:
#! /bin/bash -e
: "${JENKINS_WAR:="/usr/share/jenkins/jenkins.war"}"
: "${JENKINS_HOME:="/var/jenkins_home"}"
touch "${COPY_REFERENCE_FILE_LOG}" || { echo "Can not write to ${COPY_REFERENCE_FILE_LOG}. Wrong volume permissions?"; exit 1; }
echo "--- Copying files at $(date)" >> "$COPY_REFERENCE_FILE_LOG"
find /usr/share/jenkins/ref/ \( -type f -o -type l \) -exec bash -c '. /usr/local/bin/jenkins-support; for arg; do copy_reference_file "$arg"; done' _ {} +
# if `docker run` first argument start with `--` the user is passing jenkins launcher arguments
if [[ $# -lt 1 ]] || [[ "$1" == "--"* ]]; then
# read JAVA_OPTS and JENKINS_OPTS into arrays to avoid need for eval (and associated vulnerabilities)
java_opts_array=()
while IFS= read -r -d '' item; do
java_opts_array+=( "$item" )
done < <([[ $JAVA_OPTS ]] && xargs printf '%s\0' <<<"$JAVA_OPTS")
readonly agent_port_property='jenkins.model.Jenkins.slaveAgentPort'
if [ -n "${JENKINS_SLAVE_AGENT_PORT:-}" ] && [[ "${JAVA_OPTS:-}" != *"${agent_port_property}"* ]]; then
java_opts_array+=( "-D${agent_port_property}=${JENKINS_SLAVE_AGENT_PORT}" )
fi
if [[ "$DEBUG" ]] ; then
java_opts_array+=( \
'-Xdebug' \
'-Xrunjdwp:server=y,transport=dt_socket,address=5005,suspend=y' \
)
fi
jenkins_opts_array=( )
while IFS= read -r -d '' item; do
jenkins_opts_array+=( "$item" )
done < <([[ $JENKINS_OPTS ]] && xargs printf '%s\0' <<<"$JENKINS_OPTS")
exec java -Duser.home="$JENKINS_HOME" "${java_opts_array[#]}" -jar ${JENKINS_WAR} "${jenkins_opts_array[#]}" "$#"
fi
# As argument is not jenkins, assume user want to run his own process, for example a `bash` shell to explore this image
exec "$#"
From above script, you can see, if you add CMD ["supercronic", "/etc/crontab"] to your own dockerfile, then when your container starts, it equals to execute next:
/usr/local/bin/jenkins.sh "supercronic" "/etc/crontab"
As if [[ $# -lt 1 ]] || [[ "$1" == "--"* ]]; then not match, it will directly execute the exec "$# at the last line, which results in the jenkins start code never execute.
To fix it, you had to use your own docker-entrypoint.sh to override its default entrypoint:
docker-entrypoint.sh:
#!/bin/bash
supercronic /etc/crontab &
/usr/local/bin/jenkins.sh
Dockerfile:
FROM jenkinsci/blueocean:1.17.0
USER root
ENV SUPERCRONIC_URL=https://github.com/aptible/supercronic/releases/download/v0.1.9/supercronic-linux-amd64 \
SUPERCRONIC=supercronic-linux-amd64 \
SUPERCRONIC_SHA1SUM=5ddf8ea26b56d4a7ff6faecdd8966610d5cb9d85
RUN curl -fsSLO "$SUPERCRONIC_URL" \
&& echo "${SUPERCRONIC_SHA1SUM} ${SUPERCRONIC}" | sha1sum -c - \
&& chmod +x "$SUPERCRONIC" \
&& mv "$SUPERCRONIC" "/usr/local/bin/${SUPERCRONIC}" \
&& ln -s "/usr/local/bin/${SUPERCRONIC}" /usr/local/bin/supercronic
ADD crontab /etc/crontab
COPY docker-entrypoint.sh /
RUN chmod +x /docker-entrypoint.sh
ENTRYPOINT ["/sbin/tini", "--", "/docker-entrypoint.sh"]