If you want to use Composer, be aware that 'today' Composer supports Fabric 1.1 and you are looking at Fabric 1.2 docs, and Will likely download the 'latest' Fabric 1.2 unless you specifically do something different. (Composer support for Fabric 1.2 is coming shortly. Keep an out out for the Release notes for the latest information.)
There is a Docker container for the Composer CLI which will enable you to deploy your BNA.
There are Composer tutorials on creating and deploying BNAs, but they start with the assumption that the Composer CLI is installed locally, not in a Container so you will need to adapt the approach. The key to connecting Composer to the Fabric is the connection profile, so you will need to understand the addressing and networking aspects of your Docker environment to build the right connection profile.