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