To run composer, do:
docker run --rm --user $(id -u):$(id -g) -it --volume ${COMPOSER_JSON_LOCATION}:/var/www --volume /tmp/.composer-cache:/tmp/.composer nicodocker91/composer <composer-command>
The volumes availables to mount are /var and /tmp/.composer.
The first one is related to the workdir.
The second one is for the cache of composer.
It is strongly encouraged to mount a local common volume dedicated to your composer cache, such as
/tmp/.composer-cache.
The entrypoint is /var/www.