Jenkins Docker image, to use bind mounts or not? - docker

Question

I am reading through this bit of the Jenkins Docker README and there seems to be a section that contradicts itself from my current understanding.
https://github.com/jenkinsci/docker/blob/master/README.md
It seems to me that is says to NOT use a bind mount, and then says that using a bind mount is highly recommended?
NOTE: Avoid using a bind mount from a folder on the host machine into /var/jenkins_home, as this might result in file permission
issues (the user used inside the container might not have rights to
the folder on the host machine). If you really need to bind mount
jenkins_home, ensure that the directory on the host is accessible by
the jenkins user inside the container (jenkins user - uid 1000) or use
-u some_other_user parameter with docker run.
docker run -d -v jenkins_home:/var/jenkins_home -p 8080:8080 -p
50000:50000 jenkins/jenkins:lts this will run Jenkins in detached mode
with port forwarding and volume added. You can access logs with
command 'docker logs CONTAINER_ID' in order to check first login
token. ID of container will be returned from output of command above.
Backing up data
If you bind mount in a volume - you can simply back up
that directory (which is jenkins_home) at any time.
This is highly recommended. Treat the jenkins_home directory as you would a database - in Docker you would generally put a database on
a volume.
Do you use bind mounts? Would you recommend them? Why or why not? The documentation seems to be ambiguous.

Answer 1

As commented, the syntax used is for a volume:
docker run -d -v jenkins_home:/var/jenkins_home -n jenkins ...
That defines a Docker volume names jenkins_homes, which will be created in:
/var/lib/docker/volumes/jenkins_home.
The idea being that you can easily backup said volume:
$ mkdir ~/backup
$ docker run --rm --volumes-from jenkins -v ~/backup:/backup ubuntu bash -c “cd /var/jenkins_home && tar cvf /backup/jenkins_home.tar .”
And reload it to another Docker instance.
This differs from bind-mounts, which does involve building a new Docker image, in order to be able to mount a local folder owner by your local user (instrad of the default user defined in the official Jenkins image: 1000:1000)
FROM jenkins/jenkins:lts-jdk11
USER root
ENV JENKINS_HOME /var/lib/jenkins
ENV COPY_REFERENCE_FILE_LOG=/var/lib/jenkins/copy_reference_file.log
RUN groupmod -g <yourId>jenkins
RUN usermod -u <yourGid> jenkins
RUN mkdir "${JENKINS_HOME}"
RUN usermod -d "${JENKINS_HOME}" jenkins
RUN chown jenkins:jenkins "${JENKINS_HOME}"
VOLUME /var/lib/jenkins
USER jenkins
Note that you have to declare a new volume (here /var/lib/jenkins), because, as seen in jenkinsci/docker issue 112, the official /var/jenkins_home path is already declared as a VOLUME in the official Jenkins image, and you cannot chown or chmod it.
The advantage of that approach would be to see the content of Jenkins home without having to use Docker.
You would run it with:
docker run -d -p 8080:8080 -p 50000:50000 \
--mount type=bind,source=/my/local/host/jenkins_home_dev1,target=/var/lib/jenkins \
--name myjenkins \
myjenkins:lts-jdk11-2.190.3
sleep 3
docker logs --follow --tail 10 myjenkins