I'd buy a mini PC on craigslist for $100-200. You can usually get one with 16gb of ram and i5, a 500gb ssd for that. Install a debian based linux on it (ubuntu is very common and common is good for training) Be sure to get one with the power adapter if you go ebay.

Configure it fresh from install using ansible. Prefer ansible built-ins when possible. If you're calling shell scripts you're probably doing it wrong. Create named user accounts and create keys for that user. grant your named user sudo nopasswd and disable password login. Named users are preferable for security audit purposes. ("Shit who logged in at 2PM on tuesday and deleted critical resources?", "dunno it was the 'ubuntu' user", "great I wish we logged in as ourselves so we at least had some way of knowing who's account got compromised")
Learn how to troubleshoot ssh login problems (-v), did you know that you can't log in if the permissions on the authorized keys file are wrong?

Enable and configure unattended-upgrades. Learn how UFW works and learn how to show iptables rules. Learn how to check which services are running, which are listening, which tcp connections are active.
Learn how to write a systemd start/stop script (it's like 5 lines, super easy)
Learn how to tail and grep logs to diagnose problems. Learn how to use find and grep.

When using ssl you'll probably use let's encrypt and you'll probably want to renew automagically with certbot and dns verification.

Create an elk stack, ship some logs to it. (maybe loki too)
Hold off on k8s for now. It is advanced and you're wasting your time and your most precious resource (your ability to work through frustration) if you beat your head against it too early. When you learn k8s assume the cluster is already set up and learn to deploy a single service (there's minikube and k3s for this sort of thing). Anyone running k8s already has a lot of k8s skills (or they shouldn't be using k8s to begin with). Contribute in other ways. If they don't have a lot of k8s skills and they're using it 1) they don't need a junior, they need a senior and 2) they need to get onto something simple and easy like ecs. And if they already screwed up that bad they probably need to move to heroku. (but i digress)

Run through some command line practice, learn awk, sed, the general way commands are structured (command, options, arguments). Learn to chain commands. Learn vi (did you know you can call arbitrary shell commands?). Learn how to replace lines in config files with sed, learn how to concatenate content to the end of a file, learn how to do that for a file owned by root (hint, look into tee)

Learn how to deploy code with github actions

Learn Terraform
Take the free online AWS solutions architect training courses and begin taking practice exams. The other clouds are clones. Azure with a preference for active directory, ghoogle with a preference for unnecessary complexity and pedantic bullshit.

Skip configuring a mail server, if you're doing that you're probably doing it wrong. (hint, you'll be hitting a mail sending service. Several of the small ones have a free tier which is great for learning). You'll need some more info here but you can search it up.

When learning bash scripting focus on readability and maintainability.
The Google style guide is a great reference.
https://google.github.io/styleguide/shellguide.html
In bash there are 5 ways to do something, the best way is the most readable way.
The person who thanks you may very well be yourself.

While we're on the topic, Google's SRE book is free online and epic. Learn the section on postmortems backwards and forwards. If you got a job and only brought the google postmortem template and culture with you you could improve nearly every company in the world.
Same with improving their code testing and deployment.
Same with doing simple cloud security and cost accountability tasks (like reviewing network perimeter and cost control suggestions in trusted advisor, finding unpatched servers and patching them, resolving dependbot vulnerability notices, etc.)