Containers are an instance of the Docker Image you specify and the first image listed in your configuration is the primary container image in which all steps run.
In this example, all steps run in the container created by the first image listed under the build job
If you experience increases in your run times due to installing additional tools during execution, it is best practice to use the Building Custom Docker Images Documentation to create a custom image with tools that are pre-loaded in the container to meet the job requirements.