docs: add documentation for docker-based installation

docs/getting-started/install/docker.md

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+# Docker Installation Notes
+
+Pull the dokku/dokku image:
+
+```shell
+docker pull dokku/dokku:v0.16.2
+```
+
+Next, run the image.
+
+```shell
+docker run \
+  --env DOKKU_HOSTNAME=dokku.me \
+  --name dokku \
+  --publish 3022:22 \
+  --publish 8080:80 \
+  --publish 8443:443 \
+  --volume /mount:/mount \
+  --volume /var/run/docker.sock:/var/run/docker.sock \
+  dokku/dokku:v0.16.2
+```
+
+Dokku is run with the following options:
+
+- The global hostname is set to `dokku.me` on boot.
+- The container name is dokku.
+- SSH port 22 is exposed on the host as 3022.
+- HTTP port 80 is exposed on the host as 8000.
+- HTTPS port 443 is exposed on the host as 8443.
+- Data within the container is stored on the host within the `/mount` directory.
+- The docker socket is mounted into container
+
+Application repositories, plugin config, and plugin data is persisted to disk within the specified host directory for `/mount`.
+
+To install custom plugins, create a `plugin-list` file in the host's `/mount` directory. The plugins listed herein will be automatically installed by Dokku on container boot. This file should be the following format:
+
+```
+plugin_name: repository_url
+```
+
+An example for installing the postgres and redis plugins follows:
+
+```shell
+postgres: https://github.com/dokku/dokku-postgres.git
+redis: https://github.com/dokku/dokku-redis.git
+```
+