Warning! Before using the composer option, you need to configure the template layout.
The configuration must be done in the sonata_page.templates section.

See a complete exemple here:

sonata_page:
    # [...]
    templates:
        default:
            path: 'ApplicationSonataPageBundle::demo_layout.html.twig'
            name: 'default'
            containers:
                header:
                    name: Header
                content_top:
                    name: Top content
                content:
                    name: Main content
                content_bottom:
                    name: Bottom content
                footer:
                    name: Footer
            matrix:
                layout: |
                    HHHHHHHH
                    TTTTBBBB
                    TTTTBBBB
                    TTTTBBBB
                    TTTTBBBB
                    CCCCCCCC
                    CCCCCCCC
                    FFFFFFFF

                mapping:
                    H: header
                    T: content_top
                    C: content
                    B: content_bottom
                    F: footer