Add an official image for Plone (#484)

* Add Plone
* Fix markdown fmt
* Update documentation about data storage
* Fix markdown fmt
* Add supported environment variables.
* Fix site.cfg path
* Update Plone documentation based on latest changes within repo.
* Fix usage examples
* Fix Add-ons example
* Update docs with info about ZEO cluster
* Revert update.sh
* Add required github-repo
* Add supported environment variables
* Add some credits
* Fix github users links
* Markdown fmt

plone/README-short.txt

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+Plone is a free and open source content management system built on top of Zope.

plone/content.md

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+# What is Plone?
+
+[Plone](https://plone.org) is a free and open source content management system built on top of the Zope application server.
+
+%%LOGO%%
+
+## Features
+
+-	Images for Plone 5 and Plone 4
+-	Enable add-ons via environment variables
+
+## Usage
+
+### Start a single Plone instance
+
+This will download and start the latest Plone 5 container, based on [Debian](https://www.debian.org/).
+
+```console
+$ docker run -p 8080:8080 plone
+```
+
+This image includes `EXPOSE 8080` (the Plone port), so standard container linking will make it automatically available to the linked containers. Now you can add a Plone Site at http://localhost:8080 - default Zope user and password are `admin/admin`.
+
+### Start Plone within a ZEO cluster
+
+Start ZEO server
+
+```console
+$ docker run --name=zeo plone zeoserver
+```
+
+Start 2 Plone clients
+
+```console
+$ docker run --link=zeo -e ZEO_ADDRESS=zeo:8100 -p 8081:8080 plone
+$ docker run --link=zeo -e ZEO_ADDRESS=zeo:8100 -p 8082:8080 plone
+```
+
+### Start Plone in debug mode
+
+You can also start Plone in debug mode (`fg`) by running
+
+```console
+$ docker run -p 8080:8080 plone fg
+```
+
+### Add-ons
+
+You can enable Plone add-ons via the `PLONE_ADDONS` environment variable
+
+```console
+$ docker run -p 8080:8080 -e PLONE_ADDONS="eea.facetednavigation Products.PloneFormGen" plone
+```
+
+For more information on how to extend this image with your own custom settings, adding more add-ons, building it or mounting volumes, please refer to our [documentation](https://github.com/plone/plone.docker/blob/master/docs/usage.rst).
+
+### Supported Environment Variables
+
+The Plone image uses several environment variable that allow to specify a more specific setup.
+
+-	`PLONE_ADDONS`, `ADDONS` - Customize Plone via Plone add-ons using this environment variable
+-	`PLONE_ZCML`, `ZCML` - Include custom Plone add-ons ZCML files
+-	`PLONE_DEVELOP`, `DEVELOP` - Develop new or existing Plone add-ons
+-	`ZEO_ADDRESS` - This environment variable allows you to run Plone image as a ZEO client.
+-	`ZEO_READ_ONLY` - Run Plone as a read-only ZEO client. Defaults to `off`.
+-	`ZEO_CLIENT_READ_ONLY_FALLBACK` - A flag indicating whether a read-only remote storage should be acceptable as a fallback when no writable storages are available. Defaults to `false`.
+-	`ZEO_SHARED_BLOB_DIR` - Set this to on if the ZEO server and the instance have access to the same directory. Defaults to `off`.
+-	`ZEO_STORAGE` - Set the storage number of the ZEO storage. Defaults to `1`.
+-	`ZEO_CLIENT_CACHE_SIZE` - Set the size of the ZEO client cache. Defaults to `128MB`.
+-	`ZEO_PACK_KEEP_OLD` - Can be set to false to disable the creation of `*.fs.old` files before the pack is run. Defaults to true.
+-	`HEALTH_CHECK_TIMEOUT` - Time in seconds to wait until health check starts. Defaults to `1` second.
+-	`HEALTH_CHECK_INTERVAL` - Interval in seconds to check that the Zope application is still healthy. Defaults to `1` second.
+
+## Documentation
+
+Full documentation for end users can be found in the ["docs"](https://github.com/plone/plone.docker/tree/master/docs) folder. It is also available online at http://docs.plone.org/
+
+## Credits
+
+This docker image was originally financed by the [European Environment Agency](http://eea.europa.eu), an agency of the European Union.
+
+Thanks to [Antonio De Marinis](https://github.com/demarant), [Sven Strack](https://github.com/svx) and [Alin Voinea](https://github.com/avoinea) for their preliminary work.

plone/github-repo

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+https://github.com/plone/plone.docker

plone/license.md

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+View [license information](https://plone.org/foundation/copyrights/license-faq) for the software contained in this image.