What is CodeBuild?
It is fully managed build service. You don’t need to spend time in provisioning the hardware or any VM to setup. You just have to focus on the Build logic.

CodeBuild – Docker
Code build leverages Docker under the hood. This means you are free to use millions of docker image for your testing.

How
Build instructions are mentioned in the buildspec.yml file.

Source providers
CodeCommit – GitHub – S3 etc