Seperate cluster config from images (#871)

Author: vishalbollu

docs/cluster-management/config.md

@@ -5,8 +5,6 @@ _WARNING: you are on the master branch, please refer to the docs on the branch t
 The Cortex cluster may be configured by providing a configuration file to `cortex cluster up` or `cortex cluster update` via the `--config` flag (e.g. `cortex cluster up --config=cluster.yaml`). Below is the schema for the cluster configuration file, with default values shown (unless otherwise specified):
 
 <!-- CORTEX_VERSION_MINOR -->
-<!-- CORTEX_VERSION_BRANCH_STABLE -->
-
 ```yaml
 # cluster.yaml
 
@@ -48,7 +46,14 @@ log_group: cortex
 # whether to use spot instances in the cluster (default: false)
 # see https://cortex.dev/v/master/cluster-management/spot-instances for additional details on spot configuration
 spot: false
+```
 
+<!-- CORTEX_VERSION_BRANCH_STABLE -->
+The docker images used by Cortex are listed below. They can be overridden to use custom images by specifying them in your cluster configuration file.
+
+You can follow these [instructions](../deployments/system-packages.md) to build and push custom Docker images to a registry and configure Cortex to use them.
+
+```yaml
 # docker image paths
 image_python_serve: cortexlabs/python-serve:master
 image_python_serve_gpu: cortexlabs/python-serve-gpu:master

docs/deployments/system-packages.md

@@ -14,7 +14,7 @@ mkdir my-api && cd my-api && touch Dockerfile
 
 The Docker images used to deploy your models are listed below. Based on the Cortex Predictor and compute type specified in your API configuration, choose a Cortex image to use as the base for your custom Docker image.
 
-### Base Cortex images
+### Base Cortex images for model serving
 
 <!-- CORTEX_VERSION_BRANCH_STABLE x5 -->
 * Python (CPU): cortexlabs/python-serve:master