Include the Plugin Installation Manager Tool into the Vanilla base image

[mostafahussein]

This PR adds jenkins-plugin-manager to the final jenkinsfile-runner image which gives the ability to install custom plugins afterwards by using the image as base like the following:

FROM jenkins-runner-vanilla:latest
COPY plugins.txt /usr/share/jenkins/ref/plugins.txt
RUN cd /app/jenkins && jar -cvf jenkins.war *
RUN java -jar /app/bin/jenkins-plugin-manager.jar --war /app/jenkins/jenkins.war --plugin-file /usr/share/jenkins/ref/plugins.txt && rm /app/jenkins/jenkins.war

This will close #190 and close #189 after adding the documentation to this PR
Notes:
I couldn't make jenkins-plugin-manager to use the /app/jenkins directly like the runner command line can.
So can we make this possible in order to get rid of the extra line where I package the directory to war file?

Also, I have noticed this message:

Will use war file: /app/jenkins/jenkins.war
Unable to get version from war file

CC @stopalopa

[oleg-nenashev]

Thanks for your pull request!
Would it be possible to also add a small demo describing how to use it in downstream images?

[oleg-nenashev]

I wonder why it is located in POM instead of CLI. CC @stopalopa

[mostafahussein]

Do you mean that we should move these lines to be installed through the POM ?

[oleg-nenashev]

Sorry, missed the ping. No, no action required here. It is rather about the GitHub releases deployment

[mostafahussein]

I will work on this PR today to resolve the conflict and add an example to the docs

[mostafahussein]

@oleg-nenashev Dockerfile and docs updated. Can you please review it again ? note that the current conflict because I have modified steps that should be added - if you approved - instead of the master version

[oleg-nenashev]

Documentation needs to be moved/enhanced before the release, but I will do it in a follow-up