fix: add default value for TARGETARCH ARG

Give TARGETARCH a default of 'amd64' so the azcopy download URL
is always valid even when not running under BuildKit/buildx.

Author: andyzhangx

pkg/azurefileplugin/Dockerfile

@@ -20,7 +20,7 @@ FROM ${BASE_IMAGE} AS base
 
 FROM --platform=$BUILDPLATFORM ${BASE_IMAGE} AS builder
 
-ARG TARGETARCH
+ARG TARGETARCH=amd64
 
 RUN apt update \
     && apt install -y curl \