Merge pull request apache#81 from mesosphere/document-custom-build

Document building a custom image

Author: Suzanne Scala

docs/user-docs.md

@@ -563,6 +563,33 @@ To set Spark properties with a configuration file, create a
 `spark-defaults.conf` file and set the environment variable
 `SPARK_CONF_DIR` to the containing directory. [Learn more][15].
 
+## Custom Docker Image
+
+Note: custom docker images are not supported by Mesosphere.
+
+You can customize the Docker image in which Spark runs by extending
+the standard Spark Docker image. In this way, you can install your own
+libraries, such as a custom Python library.
+
+1. In your Dockerfile, extend from the standard Spark image and add your
+customizations:
+
+    ```
+    FROM mesosphere/spark:1.0.4-2.0.1
+    RUN apt-get install -y python-pip
+    RUN pip install requests
+    ```
+
+1. Then build an image from your Dockerfile.
+
+        $ docker build -t username/image:tag .
+        $ docker push username/image:tag
+
+1. Reference your custom Docker image with the `--docker-image` option
+when running a Spark job.
+
+        $ dcos spark run --docker-image=myusername/myimage:v1 --submit-args="http://external.website/mysparkapp.py 30"
+
 <a name="pysparkshell"></a>
 # Interactive Spark Shell