As you are using Slackware, using the Slackbuild from SBo would definitely be the better option, indeed.

Anyways, in general containers (not only Docker, but also LXC, systemd-nspawn, ...) are nice if you use them correctly. They are nice for sandboxing: don't trust that Steam binary (which you shouldn't, there were some ridiculous bugs)? Just put it in a container. The same for Skype or any other untrusted software.
I also use them for testing, it is easy to just test something new in a container without the overhead of a VM. Or for running something old with setting up an old distribution in a container.

TobiSGD, how do you sandbox a graphical application in a container? That's not something I've done before.

That is actually pretty easy, you just have to bind-mount to the container the needed files/devices, look here for example:
https://www.stgraber.org/2014/02/09/...in-containers/
https://maci0.wordpress.com/2014/05/...am-pidgin-vlc/
https://github.com/noisecapella/docker-steam-pulseaudio

This is not as secure as sandboxing servers, though, since the application needs access to the graphics card, X server or sound device, but still prevents the application from accessing anything else outside of the container or accidentally delete your /home directory (happened for some people with Steam due to sloppy Bash coding by Valve developers not long ago).