Add windows flavor commands for docker

[prestonvanloon]

Windows users have experienced issues with new line escapes.

Instead of this format:

docker run -it -v /tmp/prysm-data:/data \
   gcr.io/prysmaticlabs/prysm/validator:sapphire \
   accounts create --keystore-path=/data --password=changeme

Display this for windows:

docker run -it -v /tmp/prysm-data:/data gcr.io/prysmaticlabs/prysm/validator:sapphire  accounts create --keystore-path=/data --password=changeme

Otherwise only the first line runs and the user is often confused / stuck.

[har00ga]

Related: OffchainLabs/prysm#3997