plugins: add example docs for a rexray plugin

Signed-off-by: Tibor Vass <tibor@docker.com>

docs/extend/EBS_volume.md

@@ -0,0 +1,164 @@
+---
+description: Volume plugin for Amazon EBS
+keywords: "API, Usage, plugins, documentation, developer, amazon, ebs, rexray, volume"
+title: Volume plugin for Amazon EBS
+---
+
+<!-- This file is maintained within the docker/docker Github
+     repository at https://github.com/docker/docker/. Make all
+     pull requests against that repo. If you see this file in
+     another repository, consider it read-only there, as it will
+     periodically be overwritten by the definitive file. Pull
+     requests which include edits to this file in other repositories
+     will be rejected.
+-->
+
+# A proof-of-concept Rexray plugin
+
+In this example, a simple Rexray plugin will be created for the purposes of using
+it on an Amazon EC2 instance with EBS. It is not meant to be a complete Rexray plugin.
+
+The example source is available at [https://github.com/tiborvass/rexray-plugin](https://github.com/tiborvass/rexray-plugin).
+
+To learn more about Rexray: [https://github.com/codedellemc/rexray](https://github.com/codedellemc/rexray)
+
+## 1. Make a Docker image
+
+The following is the Dockerfile used to containerize rexray.
+
+```Dockerfile
+FROM debian:jessie
+RUN apt-get update && apt-get install -y --no-install-recommends wget ca-certificates
+RUN wget https://dl.bintray.com/emccode/rexray/stable/0.6.4/rexray-Linux-x86_64-0.6.4.tar.gz -O rexray.tar.gz && tar -xvzf rexray.tar.gz -C /usr/bin && rm rexray.tar.gz
+RUN mkdir -p /run/docker/plugins /var/lib/libstorage/volumes
+ENTRYPOINT ["rexray"]
+CMD ["--help"]
+```
+
+To build it you can run `image=$(cat Dockerfile | docker build -q -)` and `$image`
+will reference the containerized rexray image.
+
+## 2. Extract rootfs
+
+```sh
+$ TMPDIR=/tmp/rexray  # for the purpose of this example
+$  # create container without running it, to extract the rootfs from image
+$ docker create --name rexray "$image"
+$  # save the rootfs to a tar archive
+$ docker export -o $TMPDIR/rexray.tar rexray
+$  # extract rootfs from tar archive to a rootfs folder
+$ ( mkdir -p $TMPDIR/rootfs; cd $TMPDIR/rootfs; tar xf ../rexray.tar )
+```
+
+## 3. Add plugin configuration
+
+We have to put the following JSON to `$TMPDIR/config.json`:
+
+```json
+{
+      "Args": {
+        "Description": "",
+        "Name": "",
+        "Settable": null,
+        "Value": null
+      },
+      "Description": "A proof-of-concept EBS plugin (using rexray) for Docker",
+      "Documentation": "https://github.com/tiborvass/rexray-plugin",
+      "Entrypoint": [
+        "/usr/bin/rexray", "service", "start", "-f"
+      ],
+      "Env": [
+        {
+          "Description": "",
+          "Name": "REXRAY_SERVICE",
+          "Settable": [
+            "value"
+          ],
+          "Value": "ebs"
+        },
+        {
+          "Description": "",
+          "Name": "EBS_ACCESSKEY",
+          "Settable": [
+            "value"
+          ],
+          "Value": ""
+        },
+        {
+          "Description": "",
+          "Name": "EBS_SECRETKEY",
+          "Settable": [
+            "value"
+          ],
+          "Value": ""
+        }
+      ],
+      "Interface": {
+        "Socket": "rexray.sock",
+        "Types": [
+          "docker.volumedriver/1.0"
+        ]
+      },
+      "Linux": {
+        "AllowAllDevices": true,
+        "Capabilities": ["CAP_SYS_ADMIN"],
+        "Devices": null
+      },
+      "Mounts": [
+        {
+          "Source": "/dev",
+          "Destination": "/dev",
+          "Type": "bind",
+          "Options": ["rbind"]
+        }
+      ],
+      "Network": {
+        "Type": "host"
+      },
+      "PropagatedMount": "/var/lib/libstorage/volumes",
+      "User": {},
+      "WorkDir": ""
+}
+```
+
+Please note a couple of points:
+- `PropagatedMount` is needed so that the docker daemon can see mounts done by the
+rexray plugin from within the container, otherwise the docker daemon is not able
+to mount a docker volume.
+- The rexray plugin needs dynamic access to host devices. For that reason, we
+have to give it access to all devices under `/dev` and set `AllowAllDevices` to
+true for proper access.
+- The user of this simple plugin can change only 3 settings: `REXRAY_SERVICE`,
+`EBS_ACCESSKEY` and `EBS_SECRETKEY`. This is because of the reduced scope of this
+plugin. Ideally other rexray parameters could also be set.
+
+## 4. Create plugin
+
+`docker plugin create tiborvass/rexray-plugin "$TMPDIR"` will create the plugin.
+
+```sh
+$ docker plugin ls
+ID                  NAME                             DESCRIPTION                         ENABLED
+2475a4bd0ca5        tiborvass/rexray-plugin:latest   A rexray volume plugin for Docker   false
+```
+
+## 5. Test plugin
+
+```sh
+$ docker plugin set tiborvass/rexray-plugin EBS_ACCESSKEY=$AWS_ACCESSKEY EBS_SECRETKEY=$AWS_SECRETKEY`
+$ docker plugin enable tiborvass/rexray-plugin
+$ docker volume create -d tiborvass/rexray-plugin my-ebs-volume
+$ docker volume ls
+DRIVER                              VOLUME NAME
+tiborvass/rexray-plugin:latest      my-ebs-volume
+$ docker run --rm -v my-ebs-volume:/volume busybox sh -c 'echo bye > /volume/hi'
+$ docker run --rm -v my-ebs-volume:/volume busybox cat /volume/hi
+bye
+```
+
+## 6. Push plugin
+
+First, ensure you are logged in with `docker login`. Then you can run:
+`docker plugin push tiborvass/rexray-plugin` to push it like a regular docker
+image to a registry, to make it available for others to install via
+`docker plugin install tiborvass/rexray-plugin EBS_ACCESSKEY=$AWS_ACCESSKEY EBS_SECRETKEY=$AWS_SECRETKEY`.

docs/extend/index.md

@@ -154,7 +154,7 @@ This plugin is a volume driver. It requires a `host` network and the
 entrypoint and uses the `/run/docker/plugins/sshfs.sock` socket to communicate
 with Docker Engine. This plugin has no runtime parameters.
 
-### Creating the plugin
+#### Creating the plugin
 
 A new plugin can be created by running
 `docker plugin create <plugin-name> ./path/to/plugin/data` where the plugin