Containers distros supports Slackware 15

[Giantuz]

Succesfully tested Slackware 15 (current) as a supported container image:

• Base Image: docker.io/vbatts/slackware:current
• Additional info: Had to use a custom image to ensure basic dependencies were met as I was having problems with the creation of the distrobox container after distrobox-init (haven't got much time to dig further)...but it works :)

Great project!