So assuming , you will be using a single domain with 400 sub-domains, you could use an ansible role/playbook. that will
- check-out code
- build your docker image, start the container on some port
- register your docker with nginx and create a virtual host
- or register them with ALB and create a listener
- make a subdomain for you app/branch
you can have branch-name, docker port and sub-domain as variable and define them at run time
PS : Not the best solution, but could work for your use case