Hi, Why install docker-compose inside the docker build ..? When it's used..?

[EOM]

Hello, a question I don't understand why install docker-compose inside the docker build ..? When it's used..?

docker-github-runner/docker/Dockerfile:
`....
// Install Docker CLI
RUN curl -fsSL https://get.docker.com -o- | sh &&
rm -rf /var/lib/apt/lists/* &&
apt-get clean

// Install Docker-Compose
RUN curl -L -o /usr/local/bin/docker-compose
"https://github.com/docker/compose/releases/download/${DOCKER_COMPOSE_VERSION}/docker-compose-$(uname -s)-$(uname -m)" &&
chmod +x /usr/local/bin/docker-compose
....
`