How does ENTRYPOINT Docker directive react when extending images

Question

I want to know does react the ENTRYPOINT instruction from Dockerfiles when heritage happens :

Let's say for example I have an image called : jenkins

FROM java:8-jdk
RUN ...
ENTRYPOINT ["/bin/tini", "--", "/usr/local/bin/jenkins.sh"]

By running this image, the ENTRYPOINT directive will start and install the application as expected

Let's say now that I want to extend this image with a new Dockerfile, I call it : jenkins-custom

FROM jenkins

# enable start tls
RUN echo "JENKINS_JAVA_OPTIONS=\"-Dmail.smtp.starttls.enable=true\"" >> /etc/default/jenkins
RUN chown jenkins:docker /etc/default/jenkins

Should I consider that :

1. the jenkins entrypoint is triggered after my new lines.
2. entrypoint will be trigered before my new lines.
3. entrypoint will not be triggered.

In my example, I am trying to activate STARTTLS in default Jenkins Docker image, should I just restart the process in the second image ?

Answer 1

The commands in ENTRYPOINT run when you execute docker run. However, commands in RUN are executed when you run docker build.

In your case, what's going to happen is that when you docker build the image, a new Jenkins configuration file is generated, and then when you docker run it, tini is launched, and in turns execute the jenkins-entrypoint.sh.

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If what you're trying to do is change the Jenkins configuration and nothing else, what you have here is good.