Docker for Mac: add page on disk space

The disk space usage of Docker on Mac has been difficult to manage
and has confused lots of people, see

https://github.com/docker/for-mac/issues/371

This patch adds a top-level section to the Docker for Mac docs
alongside networking which explains

- where the space has gone
- how to reclaim some of it

In future I'd like to add more about why some systems use `Docker.raw`
and some `Docker.qcow2` but we're in a transitional state at the moment.
It'll be simpler to explain after the next stable version has been
released.

Signed-off-by: David Scott <dave.scott@docker.com>

docker-for-mac/space.md

@@ -0,0 +1,88 @@
+---
+description: Disk space
+keywords: mac, disk
+title: Disk space in Docker for Mac
+---
+
+Docker for Mac stores Linux containers and images in a single large "disk image" file
+in the Mac filesystem. This is different to Docker on Linux which usually stores containers
+and images in the `/var/lib/docker` directory.
+
+## Where is the "disk image" file?
+
+To locate the "disk image" file, first click on the whale menu and then click on
+"Preferences...". When the preferences window appears, click on the "Disk" icon
+and then "Reveal in Finder":
+
+![Disk preferences](images/settings-disk.png)
+
+The preferences window also shows how much disk space the file is currently consuming.
+In this example it is consuming only 2.4 GB out of a maximum of 64 GB.
+
+Note that other tools may display the space usage of the file incorrectly.
+
+## If the file is too big
+
+If the file is too big, you can
+- move it to a bigger drive;
+- delete unnecessary containers and images; or
+- reduce the maximum size of the file.
+
+### To move the file to a bigger drive
+
+To move the file, open the "Preferences..." menu, click on the "Disk" icon and then click
+on "Move disk image". Do not move the file directly in the finder or Docker for Mac will
+lose track of it.
+
+### To delete unnecessary containers and images
+
+To check whether you have too many unnecessary containers and images, first list images
+with:
+```bash
+$ docker image ls
+```
+then list containers with:
+```bash
+$ docker container ls -a
+```
+
+If there are lots of unneeded objects, try the command
+```bash
+$ docker system prune
+```
+This will remove all stopped containers, unused networks, dangling images and build cache.
+
+Note that it may take a few minutes before space becomes free on the host depending
+on what format the "disk image" file is in:
+- If the file is named `Docker.raw`: space on the host should be reclaimed within a few
+  seconds.
+- If the file is named `Docker.qcow2`: space will be freed by a background process after
+  a few minutes.
+
+Note that space is only freed when images are deleted. Space is not freed automatically
+when files are deleted inside running containers. To trigger a space reclamation at any
+point, use the command:
+
+```
+$ docker run --privileged --pid=host justincormack/nsenter1 /sbin/fstrim /var/lib/docker
+```
+
+Note that many tools will report the maximum file size, not the actual file size.
+To query the actual size of the file on the host from a terminal, use:
+```bash
+$ cd ~/Library/Containers/com.docker.docker/Data
+$ cd vms/0   # or com.docker.driver.amd64-linux
+$ ls -klsh Docker.raw
+2333548 -rw-r--r--@ 1 akim  staff    64G Dec 13 17:42 Docker.raw
+```
+In this example the actual size of the disk is `2333548` KB, whereas the maximum size
+of the disk is `64` GB.
+
+### To reduce the maximum size of the file
+
+To reduce the maximum size of the file, first click on the whale menu and then click on
+"Preferences...". When the preferences window appears, click on the "Disk" icon.
+The preferences window contains a slider which allows the maximum disk size to be set.
+If the maximum size is reduced, the current file will be deleted and therefore all
+containers and images will be lost.
+