Take a look at the following post and the comments that follow:
It looks like @pmusaraj is going to be working on something that will end up being exactly what you are looking for, but in the mean time he provides instructions for using svg-uri().
Edit: The Github link in the above post appears to be broken. This should be the current working link:
Once you’ve clicked on that link, then click on the appropriate set (solid in your case), then click the raw button and search the page for check-circle. The code below should be what you need, but I wanted to include instructions for posterity.
This feels to me like a very reasonable feature request for core, “allow mods to pick a font awesome icon or image per tag”, I can see it as something very useful, especially when the icon is perfect, For example a docker icon for the docker tag here