Nextcloud is a suite of client-server software for creating and using file hosting services. Nextcloud application functionally is similar to Dropbox. Unlike Dropbox, Nextcloud does not offer off-premises file storage hosting. Nextcloud is free and open-source, which means that anyone is allowed to install and operate it on their own private server devices.
Docker is a set of platform as a service (PaaS) products that uses OS-level virtualization to deliver software in packages called containers. Containers are isolated from one another and bundle their own software, libraries and configuration files; they can communicate with each other through well-defined channels. All containers are run by a single operating-system kernel and are thus more lightweight than virtual machines.