A lot of software can be installed today via docker. Would you recommend doing that instead of a normal install in a VM?
I am not planning to run more than one function per VM.
A good example is NextCloud. You can find complete images with everything needed.
What would you run as base? A minimal Ubuntu/Debian server?
Make sure you know who is maintaining the docker images because that can be an issue. Are they a trusted source? Do trust image maintainer? As for base operating system Debian or Ubuntu is fine.
A very good point!!
As a docker image is supposed to contain everything needed to run a program, I guess it is a good idea to find a Ubuntu server version with only the bare minimum? I seem to remember that something like that existed some time ago…
I ended up with using a minimal install of Debian as a base. I am now trying to find out if there is a way to make a base install of Debian that I just can replicate to save a little time 
Things to learn.
Also, I realized that if I rent some cloud instance from Linode, DigitalOcean etc. I am actually using a VM. And everyone runs Docker etc. on that, so I guess it is not a question of either or.
The few things I will start out with are actually distributed as Docker images. Seems some projects prefer this now.
One thing you could do in a video is to show how you change an existing VM. I am very used to do this for VMs in Virtualbox, but found it a little cumbersome in XCP-NG.
Not clear on what you are asking here.
On a related note, as I mentioned about trusting the images that are on Dockerhub and elsewhere… Malicious Docker Cryptomining Images Rack Up 20M Downloads | Threatpost
What exactly are you trying to set up? Usually with docker for example on Linode (I used DO however process should be similar) – setup you Ubuntu/Debian VM, install the docker ce packages from repository along with docker-compose packages. You’ll have to add the docker repository to the apt sources list since the included docker programs contained within the main repository are usually very old. Start the docker daemon as a service – the systemd files should have been installed during setup - just enable and activate the service. I usually then just start composition of my docker-compose file and let them grad the images from Docker Hub.
In terms of changing an existing VM? Like changing distributions? No sure what question is being asked here.
Add more cores or less. Add more memory - or less. Increase the disk size or decrease.
I guess a docker from the people that actually make NextCloud should be fine?
There were several questions. And if you read the thread, you will see that most is solved. The only remaining question was if it is possible to change the VM. Not the content of the VM, but the VM. The way you can in VirtualBox.