Update docs to describe how to customize repo-server (issue #772) (#778)

docs/README.md

@@ -19,4 +19,5 @@
 
 ## Other
 * [Configuring Ingress](ingress.md)
+* [Custom Tooling](custom_tools.md)
 * [F.A.Q.](faq.md)

docs/custom_tools.md

@@ -0,0 +1,35 @@
+# Custom Tooling
+
+Argo CD bundles preferred versions of its supported templating tools (helm, kustomize, ks, jsonnet)
+as part of its container images. Sometimes, it may be desired to use a specific version of a tool
+other than what Argo CD bundles. Some reasons to do this might be:
+
+* To upgrade/downgrade to a specific version of a tool due to bugs or bug fixes.
+* To install additional dependencies which to be used by kustomize's configmap/secret generators
+  (e.g. curl, vault)
+
+As the Argo CD repo-server is the single service responsible for generating Kubernetes manifests, it
+can be customized to use alternative toolchain required by your environment.
+
+The following example describes how the repo-server manifest can be customized to use a different
+version of helm than what is bundled in Argo CD:
+
+```yaml
+    spec:
+      # 1. Define an emptyDir volume which will hold the custom binaries
+      volumes:
+      - name: custom-tools
+        emptyDir: {}
+      # 2. Use an init container to download and/or copy the custom binaries into the emptyDir
+      initContainers:
+      - name: download-tools
+        image: lachlanevenson/k8s-helm:v2.10.0
+        command: [cp, /usr/local/bin/helm, /custom-tools]
+      # 3. Volume mount the custom binary to the bin directory (overriding the existing version)
+      containers:
+      - name: argocd-repo-server
+        volumeMounts:
+        - mountPath: /usr/local/bin/helm
+          name: custom-tools
+          subPath: helm
+```