I am pretty new to all CI and CircleCI. Up until now, I have worked with CircleCI 1 using a python based build tool called cumulusCI. So I might not be using correct terms - please accept my apologies.
What CumulusCI+CircleCI do for us is take the commit from github and initiate a build on a salesforce server. A working example can be found here: https://github.com/szandor72/lightningworkbench
When we commit to master, all changes are merged to open feature branches. Those feature branches initiate a feature build via cumulusci/circleci
My goal: Those feature builds run sequentially. Ideally I would be able to use parallelism and have each container do one build.
Im happy to answer any questions concerning the details of our process.