Create resumable docker download.md

After another Docker pull losing connection partway through a download (95% of 12GB this time), I spun up a virtual machine in the cloud and did the pull there, followed by docker save and downloading locally - this allows for a resumable download over SSH.

Overview:

1. Set up cloud machine with either password or SSH key access (key access example below).
2. Install docker
3. docker pull the image
4. docker save the image as compressed image using gzip
5. Download with rsync

After installing Docker, here's an example using hello-world on the virtual machine:

docker pull hello-world
docker save hello-world | gzip > hello-world.tar.gz

Then, from your home machine:

rsync -P -z -e "ssh -i [SSHKEY]" USER@SERVER:~/hello-world.tar.gz ~/Downloads/hello-world.tar.gz
docker load < ~/Downloads/hello-world.tar.gz

rsync may be cancelled with ctrl-c, and if cancelled or otherwise failing can be resumed with the same command.