Maven missing from docker container

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The docker container does not include maven. The instructions in the video merely say

Before we get started with this video, make sure you have the following things set up, as we will not cover them during the demo. You may already have maven installed on your machine, but if not, the easiest way to install it is via Jenkins, by navigating to Manage Jenkins, Global Tool Configuration, Maven, and add the maven installation. Alternatively, you might want to install maven via your operating system.

I followed those instruction for installing maven via Jenkins adding the default installer (install from Apache), assuming that either maven must be a tool which could be installed by just unpacking files into an area in the file system to which the process/account running Jenkins had write access, or that Jenkins was running as root (a very unappealing thought). I was able to install maven by getting a root shell in the docker container (docker exec -it -u root ...) but the instructions as given in the video need to be adjusted. If it is indeed possible to install maven on the server from the Jenkins web interface, please provide more explicit instructions. Otherwise, explain the steps needed to get maven installed in the docker container without the assistance of Jenkins.