Composer in docker container
Mount you folder and run
$ docker run -P -v </Your/Absolute/Path/To/laravel/folder>:/var/www spiralout/dcomposer <YourComposerCommand>
replacing <YourComposerCommand>
with any composer command, like create, update, install.
Run to create a new laravel project
$ docker run -P -v </Your/Absolute/Path/To/laravel/folder>:/var/www spiralout/dcomposer create-project laravel/laravel /var/www --prefer-dist'
Run to add new packages
$ docker run -P -v </Your/Absolute/Path/To/laravel/folder>:/var/www spiralout/dcomposer update