added post hook for multi arch images

contrib/docker/hooks/post_push

@@ -0,0 +1,5 @@
+#!/bin/bash
+curl -sLo manifest-tool "https://github.com/estesp/manifest-tool/releases/download/v1.0.3/manifest-tool-linux-amd64"
+chmod +x manifest-tool
+sed -e "s#%image_name%#$IMAGE_NAME#" manifest.template > manifest.yaml
+./manifest-tool push from-spec manifest.yaml

contrib/docker/manifest.template

@@ -0,0 +1,11 @@
+image: %image_name%
+manifests:
+  - image: %image_name%-amd64
+    platform:
+      architecture: amd64
+      os: linux
+  - image: %image_name%-arm32v7
+    platform:
+      architecture: arm
+      os: linux
+      variant: v7