Docs: Adding how to use plugin version, through docker env variable (#19924)

* Adding grafana plugin version info

It was unclear if one can specify the plugin version to the environment variable. It is possible and works fine. Added it to the plugin information.

* Update typo docs/sources/installation/docker.md

* Applied all comments

Moved the information to make it more readable, fixed typos and added missing punctuation.

* Rewording and better example

More feedback, added example text. Did not manage to get newline for the example.

* added the missing space

docs/sources/installation/docker.md

@@ -85,6 +85,8 @@ docker run \
   grafana/grafana
 ```
 
+> If you need to specify the version of a plugin, you can add it to the `GF_INSTALL_PLUGINS` environment variable. Otherwise, the latest will be assumed. For example: `-e "GF_INSTALL_PLUGINS=grafana-clock-panel 1.0.1,grafana-simple-json-datasource 1.3.5"`
+
 ## Building a custom Grafana image with pre-installed plugins
 
 In the [grafana-docker](https://github.com/grafana/grafana/tree/master/packaging/docker)  there is a folder called `custom/` which includes a `Dockerfile` that can be used to build a custom Grafana image.  It accepts `GRAFANA_VERSION` and `GF_INSTALL_PLUGINS` as build arguments.
@@ -103,6 +105,8 @@ docker run \
   grafana:latest-with-plugins
 ```
 
+> If you need to specify the version of a plugin, you can add it to the `GF_INSTALL_PLUGINS` build argument. Otherwise, the latest will be assumed. For example: `--build-arg "GF_INSTALL_PLUGINS=grafana-clock-panel 1.0.1,grafana-simple-json-datasource 1.3.5"`
+
 ## Installing Plugins from other sources
 
 > Only available in Grafana v5.3.1+