demo-app fails on arm64 hosts

[strophy]

I'm trying to learn Argo CD and was following along with the documentation and examples included in this repo. My target cluster consists of several Raspberry Pi devices. Unfortunately the demo-app image referenced here does not exist for linux/arm64 arch, resulting in my tests failing with exec /app/simple-web-app: exec format error in the logs. Would be nice to build this image for arm arch as well, thanks!