EACCESS Permission denied on SSL certificate files when starting Phoenix production server

I have added some fresh Let's Encrypt SSL certificates to my Phoenix app as described in the guide here: https://phoenixframework.readme.io/docs/configuration-for-ssl

When I try to start my server in production mode I get the following error:

23:04:11.573 [info] Running MyAppWeb.Endpoint with Cowboy using http://:::400023:04:11.652 [error] Failed to start Ranch listener MyAppWeb.Endpoint.HTTPS in :ranch_ssl:listen([certfile: '/Users/FJ/projects/elixir/my_app/_build/prod/lib/my_app/priv/ssl/cert.crt', keyfile: '/Users/FJ/projects/elixir/my_app/_build/prod/lib/my_app/priv/ssl/priv.key', port: 443]) for reason :eacces (permission denied)

I don't understand why this happens because I have given the files pretty much all read permissions possible:

ls -la priv/ssl/total 16drwxr-xr-x  4 FJ  staff   128 Apr 25 23:09 ./drwxr-xr-x  9 FJ  staff   288 Apr 25 23:09 ../-rw-r--r--  1 FJ  staff  1708 Apr 25 23:02 cert.crt-rw-r--r--  1 FJ  staff  1708 Apr 25 23:03 priv.key

I am using MacOS locally but in my production environment (Alpine Linux) I the same error.

Here is the SSL part of production Endpoint config:

  https: [port: 443,        otp_app: :my_app,        keyfile: "priv/ssl/priv.key",        certfile: "priv/ssl/cert.crt"        ]