Basic overview: Composer is used to install project dependencies and for package distribution via https://wpackagist.org/. Docker is used for local development via any running container daemon (Vagrant inlcuded for ease of use)
Used to install dependencies and you have two options to use composer [locally, docker]
Locally installed
composer install
Docker
docker-compose -f docker-composer-installer.yml up
#or
docker run -v "$(pwd)":/app composer install
To get a local setup running you have two options, docker or vagrant
Docker-Compose
docker-compose up
Vagrant
vagrant up
Some generic info here about the project