I’m not familiar with Java or Maven, but I’ll take a guess that this image is based on Ubuntu. If it’s an Oracle flavour of Linux, swap out the package manager binary accordingly!
Add a run step thus:
apt-get install maven
That will get you the distro default version. If you want something more recent, head over to the Maven website and check out their install instructions.
You can always try building a custom image containing both the base image plus Maven on top, push it to a registry, then pull it when you need it. It depends whether installing or pulling is quicker.