Added a SO post about this issue as well: https://stackoverflow.com/questions/55582092/how-to-fix-404-not-found-errors-when-sourcing-css-and-javscript-files-in-shi
You will have to install the requested font at the top of your Dockerfile. You can add it to your list “Dependencies outside of R”:
sudo apt-get install fonts-font-awesome
I have posted this answer on SO where you provided more information:
The discussion on this topic was more fruitful at Stack Overflow and a solution was found. The tl;dr version of the answer is that shiny 1.3 does not play nice with the default / suggested configuration of NGINX. The solution is either using shiny 1.2 or updating the NGINX configuration - which I assume will work but have not done yet to verify.