I was having an issue launching another PHP application on the same device with Docker Compose.

You are having this issue because you have more than one container with same hostname (php) in same network.

As the above makes clear, it was because I had copied the compose.yml and used the same service name for the PHP container in Compose and the Caddyfile. Changing php to <app-name>-php in both Compose and Caddyfile fixed it.

Photos

Compose

  php:
    image: php-with-extensions:7.4
    container_name: photos-01-php-fpm

Caddyfile

:8091 {
    root * /media/photos
    file_server
    php_fastcgi php:9000
}

List

Compose

  php:
    image: php-with-extensions:7.4
    container_name: list-01-php-fpm

Caddyfile

:8098 {
    root * /var/www/html
    file_server
    php_fastcgi php:9000
}