When using data migration, you cannot add the "container" metadata to a Content Type.

-
    type: content
    mode: create
    metadata:
        contentType: blog
        mainTranslation: eng-GB
        container: true
    location:
        parentLocationId: 2
    fields:
        -   fieldDefIdentifier: title
            languageCode: eng-GB
            value: 'Blog'

This results in a Content Type that does not have the Container flag enabled.

Similarly, when generating migrations from a Content Type that is a container (for example, the built-in Folder CT), the resulting file has no information about being a container.