Or you can prevent this from happening by running xattr -d ~/Downloads/darktable*.dmg command before mounting the image (or xattr -dr /Applications/darktable.app after installing).
To download, visit the Docker CE for Mac download page and click Get Docker.
Installation notes Linux/Unix Binary Packages From Your Package Manager You can use VirtualBox (free from Oracle) to create a virtual machine: This will basically allow you to install Ubuntu 'within' OS X.