Improve Toolbox to Desktop migration (#11297)

* Improve Toolbox to Desktop migration

Previously we recommended converting the disk at the block level.
This is a bit risky as any on-disk format change could prevent it
from working.

It's safer to explicitly `docker commit`, then `docker save` and
`docker load` containers and images.

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

* Minor style updates

* Minor style updates

Co-authored-by: Usha Mandya <47779042+usha-mandya@users.noreply.github.com>

docker-for-mac/docker-toolbox.md

@@ -167,49 +167,31 @@ machines. One solution is to use a version manager like
 
 Docker Desktop does not propose Toolbox image migration as part of its
 installer since version 18.01.0.  You can migrate existing Docker
-Toolbox images with the scripts described below. (This migration
-cannot merge images from both Docker and Toolbox: any existing Docker image is
-*replaced* by the Toolbox images.)
+Toolbox images with the steps described below.
 
-Run the following shell commands in a terminal. You need a working
-`qemu-img`; it is part of the qemu package in both MacPorts and Brew:
+In a terminal, while running Toolbox, use `docker commit` to create an image snapshot
+from a container, for each container you wish to preserve:
 
-```sh
-$ brew install qemu  # or sudo port install qemu
+```
+$ docker commit nginx
+sha256:1bc0ee792d144f0f9a1b926b862dc88b0206364b0931be700a313111025df022
 ```
 
-First, find your Toolbox disk images. You probably have just one:
-`~/.docker/machine/machines/default/disk.vmdk`.
+Next, export each of these images (and any other images you wish to keep):
 
-```sh
-$ vmdk=~/.docker/machine/machines/default/disk.vmdk
-$ file "$vmdk"
-/Users/akim/.docker/machine/machines/default/disk.vmdk: VMware4 disk image
+```
+$ docker save -o nginx.tar sha256:1bc0ee792d144f0f9a1b926b862dc88b0206364b0931be700a313111025df022
 ```
 
-Second, find out the location and format of the disk image used by your Docker
-Desktop.
+Next, when running Docker Desktop on Mac, reload all these images:
 
-```sh
-$ settings=~/Library/Group\ Containers/group.com.docker/settings.json
-$ dimg=$(sed -En 's/.*diskPath.*:.*"(.*)".*/\1/p' < "$settings")
-$ echo "$dimg"
-/Users/akim/Library/Containers/com.docker.docker/Data/vms/0/Docker.raw
+```
+$ docker load -i nginx.tar
+Loaded image ID: sha256:1bc0ee792d144f0f9a1b926b862dc88b0206364b0931be700a313111025df022
 ```
 
-In this case the format is `raw` (it could have been `qcow2`), and the location
-is `~/Library/Containers/com.docker.docker/Data/vms/0/`.
-
-Then:
-- if your format is qcow2, run
-```sh
-$ qemu-img convert -p -f vmdk -O qcow2 -o lazy_refcounts=on "$vmdk" "$dimg"
-```
-- if your format is raw, run the following command. If you are short on disk
-  space, it is likely to fail.
-```sh
-$ qemu-img convert -p -f vmdk -O raw "$vmdk" "$dimg"
-```
+Note these steps will not migrate any `docker volume` contents: these must
+be copied across manually.
 
 Finally (optional), if you are done with Docker Toolbox, you may fully
 [uninstall

docker-for-windows/docker-toolbox.md

@@ -9,29 +9,33 @@ you have them, to Docker Desktop for Windows.
 
 ## How to migrate Docker Toolbox disk images to Docker Desktop
 
-> **Warning**
->
-> Migrating disk images from Docker Toolbox _clobbers_ Docker images if they
-> exist. The migration process replaces the entire VM with your previous Docker
-> Toolbox data.
-{: .warning }
+Docker Desktop does not propose Toolbox image migration as part of its
+installer since version 18.01.0. You can migrate existing Docker
+Toolbox images with the steps described below.
 
-1.  Install [qemu](https://www.qemu.org/){: target="_blank" class="_"} (a machine emulator): [https://cloudbase.it/downloads/qemu-img-win-x64-2_3_0.zip](https://cloudbase.it/downloads/qemu-img-win-x64-2_3_0.zip).
-2.  Install [Docker Desktop for Windows](install.md){: target="_blank" class="_"}.
-3.  Stop Docker Desktop, if running.
-4.  Move your current Docker VM disk to a safe location:
+In a terminal, while running Toolbox, use `docker commit` to create an image snapshot
+from a container, for each container you wish to preserve:
 
-    ```shell
-    mv 'C:\Users\Public\Documents\Hyper-V\Virtual Hard Disks\MobyLinuxVM.vhdx' C:/<any directory>
-    ```
+```
+> docker commit nginx
+sha256:1bc0ee792d144f0f9a1b926b862dc88b0206364b0931be700a313111025df022
+```
 
-5.  Convert your Toolbox disk image:
+Next, export each of these images (and any other images you wish to keep):
 
-    ```shell
-    qemu-img.exe convert 'C:\Users\<username>\.docker\machine\machines\default\disk.vmdk' -O vhdx -o subformat=dynamic -p 'C:\Users\Public\Documents\Hyper-V\Virtual Hard Disks\MobyLinuxVM.vhdx'
-    ```
+```
+> docker save -o nginx.tar sha256:1bc0ee792d144f0f9a1b926b862dc88b0206364b0931be700a313111025df022
+```
 
-6.  Restart Docker Desktop (with your converted disk).
+Next, when running Docker Desktop on Windows, reload all these images:
+
+```
+> docker load -i nginx.tar
+Loaded image ID: sha256:1bc0ee792d144f0f9a1b926b862dc88b0206364b0931be700a313111025df022
+```
+
+Note these steps will not migrate any `docker volume` contents: these must
+be copied across manually.
 
 ## How to uninstall Docker Toolbox