Merge pull request #5740 from dokku/5739-docs

Add a note about how running plugin:install does not apply for docker-based installs

docs/advanced-usage/plugin-management.md

@@ -78,6 +78,8 @@ dokku plugin:installed postgres
 
 ### Installing a plugin
 
+> Note: Plugins installed in this method within a [docker-based installation](/docs/getting-started/install/docker.md) of Dokku do not persist across installs. Please see the [plugin installation section](/docs/getting-started/install/docker.md#plugin-installation) of the docker-based installation docs for further details.
+
 Installing a plugin is easy as well using the `plugin:install` command. This command will also trigger the `install` pluginhook on all existing plugins.
 
 The most common usage is to install a plugin from a url. This url may be any of the following:

docs/getting-started/install/docker.md

@@ -40,6 +40,8 @@ Other docker container options can also be used when running Dokku, though the s
 
 ## Plugin Installation
 
+### On Dokku start
+
 To install custom plugins, create a `plugin-list` file in the host's `/var/lib/dokku` directory. The plugins listed herein will be automatically installed by Dokku on container boot. This file should be the following format:
 
 ```yaml
@@ -53,6 +55,16 @@ postgres: https://github.com/dokku/dokku-postgres.git
 redis: https://github.com/dokku/dokku-redis.git
 ```
 
+### Prior to Dokku start
+
+The alternative is to build a custom docker image via a custom Dockerfile. This Dockerfile can run any `plugin:install` command. Note that the version installed at that time will be the one that persists. Below is an example Dockerfile showing this method.
+
+```Dockerfile
+FROM dokku/dokku:0.30.1
+RUN dokku plugin:install https://github.com/dokku/dokku-postgres.git postgres
+RUN dokku plugin:install https://github.com/dokku/dokku-redis.git redis
+```
+
 ## SSH Key Management
 
 To initialize ssh-keys within the container, use `docker exec` to enter the container and run the appropriate ssh-keys commands.