Most of the pro-Docker arguments go around security
Actually Docker and the success of containers is mostly due to the ease of shipping code that carries its own dependencies and can be run anywhere. Security is a side-effect and definitely not the reason why containers picked-up.
systemd can provide as much isolation a docker containers and 2) there are other container solutions that are at least as safe as Docker and nobody cares about them.
Yes, and it's much harder to achieve the same.
In systemd you need to use 30 different options to get what using containers you achieve almost instantly and with much less hussle. I made an example on my blog where I decided to run blocky
in Systemd and not in Docker. It's just less convenient and accessible, harder to debug and also relies on each individual user to do it, while with containers a lot gets packed into the image and therefore harder to mess up.
Docker isn’t totally proprietary
There are a many container runtimes (CRI-O, podman, mirantis, containerd, etc.). Docker is just a convenient API, containers are fully implemented just with Linux native features (namespaces, seccomp, capabilities, cgroups) and images follow an open standard (OCI).
I will avoid comment what looks like a rant, but I want to simply remind you that containers are the successor of VMs (virtualize everything!), platforms that were completely proprietary and in the hands of a handful of vendors, while containers use only native OS features and are therefore a step towards openness.
This can be absolutely true the other way around too, depending on how proficient you are, and what you are used to or find intuitive. For me, macOS is extremely unintuitive, for example, while my fully personalized Linux setup allows me to do what I want. It is very subjective, ultimately.