composer.json file exists in current directory but composer is unable to find it (Windows 10)

**command being run:**
```
docker run --rm --interactive \
--volume $PWD:/app \
--volume /etc/passwd:/etc/passwd:ro \
--volume /etc/group:/etc/group:ro \
--volume ${COMPOSER_HOME:-$HOME/.composer}:/tmp \
--user $(id -u):$(id -g) \
composer:1.8 install $@
```
**output:**

```
Composer could not find a composer.json file in /app
To initialize a project, please create a composer.json file as described in the https://getcomposer.org/ "Getting Started" section
```

**`ls` output:**
```
composer.json composer.lock
```