Unable to configure object storage

I’m trying to configure object storage on my self-managed GitLab installation, which uses the GitLab omnibus Docker image. However, it doesn’t seem to accept any of the documented object_store configurations.

The relevant docker-compose.yml section:

    gitlab:
        image: gitlab/gitlab-ee:latest
        restart: unless-stopped
        volumes:
            - "./gitlab.rb:/etc/gitlab/gitlab.rb"
            - "gitlab-config:/etc/gitlab"
            - "gitlab-logs:/var/log/gitlab"
            - "gitlab-data:/var/opt/gitlab"
        ports:
            - "9002:80"
            - "9005:5050"
            - "2222:22"

The relevant contents of gitlab.rb:

gitlab_rails['object_store']['enabled'] = true
gitlab_rails['object_store']['proxy_download'] = true
gitlab_rails['object_store']['connection'] = {
    'provider' => 'AWS',
    'region' => 'us-east-1',
    'use_iam_profile' => true,
}

Running gitlab-ctl reconfigure produces the following error:

There was an error running gitlab-ctl reconfigure:

undefined method `[]=' for nil:NilClass

The given traceback points to the line gitlab_rails['object_store']['enabled'] = true. Removing those lines causes gitlab-ctl reconfigure to succeed. This seems to imply that object_store is not a valid configuration option, despite all documentation to the contrary.

Is there something obvious that I’m missing here?