When an application container is started, the platform is using the environment
variables defined in the application configuration but is also injecting a set of
environment variables in its environment. In the case of
web containers, an
$PORT is defined.
$PORT: Port number your server has to bind on.
$CONTAINER: Type and index of the container,
$CONTAINER_VERSION: Version of the container started, usually the GIT commit SHA.
$APP: Name of the application deployed