Only build .jar on current build platform

Because the .jar is platform-independent, we don't need to build it for every architecture.

Dockerfile

@@ -1,4 +1,4 @@
-FROM openjdk:8-jre-alpine AS builder
+FROM --platform=$BUILDPLATFORM openjdk:8-jre-alpine AS builder
 
 RUN apk update && apk add gradle git && rm -rf /var/lib/apk/* /var/cache/apk/*