In Docker for Mac 1.12 the only way to free space on the host is to
delete the qcow2 which means all containers and images have to be
In Docker for Mac 1.13 there is preliminary support for shrinking the
qcow2 file non-destructively using "TRIM" (as also used on SSDs).
Unfortunately this isn't (yet) fully automatic -- it runs in the
background and requires the app to be occasionally restarted.
Related to [docker/for-mac#371]
Signed-off-by: David Scott <email@example.com>