icons are not displayed when using a prefix

Hello everybody!

Faced the problem of displaying icons if a prefix is used.
my docker-compose.yml file:
```
...
environment:
- "RAILS_RELATIVE_URL_ROOT=/fluentd"
...
```

I'm using the current master:
$ git log
```
commit a4883e7d17e17bb9ec1f4b9cc730f604e79ba1c3 (HEAD -> master, origin/master, origin/HEAD)
Merge: 9e4a92d 2d03328
Author: Kentaro Hayashi
Date: Fri Aug 13 18:17:39 2021 +0900

Merge pull request #379 from fluent/dependabot/npm_and_yarn/url-parse-1.5.3

Bump url-parse from 1.5.1 to 1.5.3
```