Previous versions of Sketch app allowed for the creation of dotted lines. While it wasn’t perfect, it was an option. I’m not sure exactly when this feature apparently became deprecated. More recent versions have removed the ability to put any number lesser than “1” in the line settings.
It would be nice if Sketch would restore support for decimals and/or “0” values in the line settings. The video below shows how this worked in Sketch app 3.3.3,
#BringBackTheZero, dots for all!