Added snippet to generate and install favicons

[b3rrytech]

My first ever PR so I hope I've done this right.

I thought the Docker container needed a Vorta favicon instead of the blue box. Followed the instructions for the baseimage from jlesage. The image link is from the Vorta README.md

Maybe it's best to instead add the icon to the repo instead of using this link, but that might need a separate PR first so the link is right?

[m3nu]

Looks correct, but it also introduces a dependency on some externally hosted site. Is there a way to have the file inside this repo?

[b3rrytech]

Yes the dependency isn’t ideal. I’ll try and do another PR and just adding the icon to this repo first. After that I should be able to change this PR with the url to that icon.

[Ranbato]

The script works with a local file so we can just add the image to this repo and call the script. Don't close this PR, just add the file in this PR and change the URL