Add variant options for OS and setup with docker

[Yoshyn]

Hi,

Over internet, I searched for a simple way to burn a raspbian image with ssh enabled. Without the painful managing of mounting stuff and so on. And find that repo. Thanks for your amazing work on it !

Sadly, i'm on mac os so tool like losetup are not available, so it was not usable. Also, i wanted a very simple way to use it and docker is my solution.

And BTW, I need to use a 'lite' version of the os.

This PR solve both of my concerns :

• Add an option to be able to choose between 'full' and 'lite' variant for raspbian os.
• Add instruction to the README to be able to create the ssh-enabled image with docker.

Note : the ugly option -v /dev:/dev seem to be needed (check here for more info). Without that, the losetup command does not successfully retrieve the partition table and create device.

Not sure if it's something that you'll want to be merged but I create the PR in case of. If someone on Mac encounter this problem : it can help. :)

[thompcd]

Thank you for this! I was using this on a mac and also began re-writing it, it was very useful

[Yoshyn]

@thompcd : Thank for your comments and Happy to see that It has been useful for someone. 😊 I just edit the PR and rebase with the last version of the project.

@kenfallon : Sorry, I know the PR is not good. I have a linter that reformat the code automatically and even without that it include three subjects (variant, ssh port & instructions) which is pretty bad for a PR. I'm not playing with raspberry since month and I've no time for that yet but feel free to take portion of code in this PR as you want. :)

In the meantime, if you don't mind, I would keep this PR open. For someone like me (on MAC) who doesn't often use tools like losetup, it's very convenient to have a simplified use.

[kenfallon]

I think everything has been implemented in version 1.5.x