Deprecate our "ownCloud" image

Upstream maintains their own image (https://owncloud.org/download/#owncloud-server-docker / https://github.com/owncloud-docker/server / https://hub.docker.com/r/owncloud/server/), and it's officially supported and recommended directly by them (so there's really no sense in us duplicating that effort without their involvement).

owncloud/deprecated.md

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+This image has been deprecated in favor of the [official `owncloud/server` image](https://hub.docker.com/r/owncloud/server/) provided, maintained, and supported by [ownCloud upstream](https://owncloud.org/download/#owncloud-server-docker). The images found here will receive no further updates after 2018-12-31 (Dec 31, 2018). Please adjust your usage accordingly.