Change Docker disk space on macOS

I installed Docker 1.13 on macOS 10.10, logged in the docker "machine" with

screen ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/tty

And did a df -h. It says I have a /dev/sda1 mounted on /var/lib/docker/overlay2 with 64 GB of disk space.

/dev/sda1                62.7G      5.6G     54.0G   9% /var/lib/docker/overlay2

I had a few builds failing for "no space left on device" so I suspect this is where all images end up going.

What is not clear to me is the following:

1. what is this /dev/sda1. Is it a virtual image disk somewhere on my mac? if so, where?


2. Suppose I am running out of space and I want to store more images. How does one increase this disk space?