shiny - Using custom fonts on shinyapps.io

I would like to use a custom font in my shiny app (on plots) on shinyapps.io. I have my Roboto-Regular.ttf in the ./www/ directory. And this is the upper portion of my app.R file:

dir.create('~/.fonts')
system("chmod +x ./www/Roboto-Regular.ttf")
system("cp ./www/Roboto-Regular.ttf ~/.fonts/")

system('fc-cache -f -v ~/.fonts/')
system('fc-match Roboto')

library(ggplot2)
library(shiny)
library(shinythemes)

library(extrafont)
font_import(pattern="Roboto",prompt=FALSE)
loadfonts()

print(fonts())

Upon deploying the app, I end up with an error that looks like this:

Registering fonts with R
Scanning ttf files in /usr/share/fonts/, ~/.fonts/ ...
Extracting .afm files from .ttf files...
/home/shiny/.fonts/Roboto-Regular.ttfWarning in gzfile(dest, "w") :
  cannot open compressed file '/opt/R/3.5.1/lib/R/library/extrafontdb/metrics/Roboto-Regular.afm.gz', probable reason 'Permission denied'

Error in value[[3L]](cond) : cannot open the connection
Calls: local ... tryCatch -> tryCatchList -> tryCatchOne -> 
Execution halted

Does anyone see what might be wrong?