Add support for distrobox

[dalingrin]

I want to suggest a new step

Which tool is this about? Where is its repository?

Distrobox Git: https://github.com/89luca89/distrobox

Which operating systems are supported by this tool?

Linux

Distrobox is a container tool similar to Toolbx, which is already supported Topgrade. Like with Toolbx, it would be nice if Topgrade could enter each Distrobox container and update using the container OS's system tools (dnf, apt-get, etc).

[misobarisic]

I would also appreciate if this were implemented.