Jenkins doesn't show me initial admin password at second build - docker

Question

Testing with Jenkins & Docker, I don't understand completely what is happening with my containers and images.
Firstly, I built my first docker container from jenkins/jenkins:tls
docker run --name myjenkins -p 8080:8080 -p 50000:50000 -v jenkins_home:/var/jenkins_home jenkins/jenkins:lts
And I received the typical messsage from jenkins installation with the initial password:
INFO:
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Jenkins initial setup is required. An admin user has been created and a password generated.
Please use the following password to proceed to installation:
xxxxxxxxxxxxxxxxxxxxxxxxxxxx
This may also be found at: /var/jenkins_home/secrets/initialAdminPassword
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I completed the installation process and I were playing with Jenkins for a while. Everything ok.
My missunderstanding come repeating the process from the beggining. I deleted my container and built the same container for a second time.
docker container stop myjenkins <- Stop container
docker container rm myjenkins <- Remove myjenkins container
docker image rm 95bf220e341a <- Remove jenkins/jenkins image
docker run --name myjenkins -p 8080:8080 -p 50000:50000 -v jenkins_home:/var/jenkins_home jenkins/jenkins:lts
But in this case, Jenkins doesn't show me a new initial password for this second time:
Jun 18, 2019 7:43:17 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory#83bb567: defining beans [authenticationManager]; root of factory hierarchy
<-- I was expecting the message just here -->
Jun 18, 2019 7:43:17 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.web.context.support.StaticWebApplicationContext#5bfdcaf3: display name [Root WebApplicationContext]; startup date [Tue Jun 18 19:43:17 UTC 2019]; root of context hierarchy
Jun 18, 2019 7:43:17 PM org.springframework.context.support.AbstractApplicationContext obtainFreshBeanFactory
INFO: Bean factory for application context [org.springframework.web.context.support.StaticWebApplicationContext#5bfdcaf3]: org.springframework.beans.factory.support.DefaultListableBeanFactory#1f98db0a
Jun 18, 2019 7:43:17 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory#1f98db0a: defining beans [filter,legacy]; root of factory hierarchy
Jun 18, 2019 7:43:18 PM jenkins.InitReactorRunner$1 onAttained
INFO: Completed initialization
Jun 18, 2019 7:43:19 PM hudson.WebAppMain$3 run
INFO: Jenkins is fully up and running
I tried with docker system prune -a but nothing changed. Every time that I tried to rebuild my container, I couldn't get the Initial Admin Password message again.
What's happening? If I delete a container.. How Docker/Jenkins knows that is not the first time I try to install jenkins?

Answer 1

-v jenkins_home:/var/jenkins_home
You're mapping that directory to somewhere so the docker image is (hopefully) immutable. Recreating it makes no difference - if you don't expunge that folder then Jenkins' config data remains intact the next time the image is started.
Also, this means your bootstrap pwd should be available on the docker host at:
jenkins_home/secrets/initialAdminPassword