Publish multi-arch alpine base

This makes functions/alpine:latest into a multi-arch image.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

sample-functions/base.yml

@@ -0,0 +1,10 @@
+provider:
+  name: openfaas
+  gateway: http://127.0.0.1:8080  # can be a remote server
+
+functions:
+  alpine-function:
+    lang: dockerfile
+    handler: ./alpinefunction
+    image: functions/alpine:latest-multiarch
+    platforms: linux/amd64,linux/arm64,linux/arm/7