Does Docker on MacOS support tmpfs?
According to the docs tmpfs is linux only. 

RAM disk

If you want "RAM only storage", you can literally use a RAM disk. ie: 

diskutil erasevolume HFS+ 'DockerDisk' `hdiutil attach -nomount ram://2097152 `


Then you can point volumes at /Volumes/DockerDisk. Drop and recreate at will. 

Docker will still store its VM and images on disk. See: Docker > Preferences > Disk.