I had the same problem. read all this information->
alankent commented 2 days ago
At the moment, yes. The new web installer is not finished yet. Only the command line install is guaranteed to work right now. In case useful, https://github.com/alankent/docker-magento2-demo-apache has a Docker image that works. Skim through the Dockerfile and the contents of the 'scripts' directory and you can see how I put it together. (Last tested with alpha100.)
alankent commented a day ago
Composer is used to download the source code, not create the database, create the local.xml file etc. If you have a look in https://github.com/alankent/docker-magento2-demo-apache/blob/master/scripts/install-magento2 you will see it runs composer, runs some scripts intended for production mode (see deploy.php and compile.php). It also runs 'composer' in the 'setup' subdirectory. The next phase can be found in https://github.com/alankent/docker-magento2-demo-apache/blob/master/scripts/runserver which creates a fresh database instance then runs the setup script (currently a part of index.php) which creates the database and sets up the local.xml file.
This is going to change and be cleaned up more before final release, but this is what I do today and it all seems to work.