Fixed Docker bootstrap capitalisation, added example bootstrap command

[ColonelPanics]

The example bootstrap file capitalises Docker which leads to the following error when trying to bootstrap:

Sanitizing environment
Building from bootstrap definition recipe
Adding base Singularity environment to container
ERROR: 'Bootstrap' type not supported: Docker

The quickstart documentation also doesn't mention how to launch a container from a bootstrap file so an example command has been added as well.

[vsoch]

looks good, thanks for the fix!