This is a Docker Compose file, which is a tool for defining and managing multi-container Docker applications. It uses YAML format to configure application's services, networks and volumes.

The version: '3.8' at the top of the file specifies the version of Docker Compose file format that is being used.

The services: section is where you define your app's services, which are the building blocks of any Docker application. Each service is an instance of a Docker image and they can communicate with each other and share data through defined networks and volumes.

In this file, there are three services defined: mainline, stable, and alpine. Each of these services is built from the same Dockerfile (nginx.Dockerfile), but with different build arguments and image names.

The build: section under each service is used to build a Docker image from a Dockerfile. The context: . line tells Docker to use the current directory as the build context. The dockerfile: nginx.Dockerfile line specifies the Dockerfile to use for building the image.

The args: section is used to define build-time variables. For the mainline and stable services, the NGINX_VERSION is set to the value of the DEFAULT_NGINX_VERSION environment variable, and the BUILD_FILE is set to debian.conf. For the alpine service, the NGINX_VERSION is set to 1.21.3 and the BUILD_FILE is set to alpine.conf.

The image: line under each service is used to specify a name and optionally a tag for the image. For example, the mainline service's image will be named app:mainline.

The depends_on: section under the mainline service specifies that this service depends on the stable service. This means that the stable service will be started before the mainline service.