add instructions to download docker on kaishi script

[erik0686]

What does this PR do?

• This PR adds instructions to optionally (if chosen) download Docker as part of the Linux script.
• This PR only adds the instructions for Linux
• I noticed that at no point does the script do an "apt update", should it, @kurenn ???

[erik0686]

Hey @kurenn! GitHub will not allow me to add reviewers, but hopefully you have some time eventually to review this PR :)

[kurenn]

It probably should, let me test this on my virtual machine, and come back to you! Thanks for the support!