docs: installing custom plugins in docker.

[skip ci]

docs/sources/installation/docker.md

@@ -87,7 +87,7 @@ docker run \
 
 ## Building a custom Grafana image with pre-installed plugins
 
-In the [grafana-docker](https://github.com/grafana/grafana-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.
+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.
 
 Example of how to build and run:
 ```bash
@@ -103,6 +103,21 @@ docker run \
   grafana:latest-with-plugins
 ```
 
+## Installing Plugins from other sources
+
+> Only available in Grafana v5.3.1+
+
+It's possible to install plugins from custom url:s by specifying the url like this: `GF_INSTALL_PLUGINS=<url to plugin zip>;<plugin name>`
+
+```bash
+docker run \
+  -d \
+  -p 3000:3000 \
+  --name=grafana \
+  -e "GF_INSTALL_PLUGINS=http://plugin-domain.com/my-custom-plugin.zip;custom-plugin" \
+  grafana/grafana
+```
+
 ## Configuring AWS Credentials for CloudWatch Support
 
 ```bash