Updated to support multiarch/BuiltKit builds

BUILDING.md

@@ -0,0 +1,5 @@
+Ensure buildx/BuildKit support is enabled and run:
+
+```
+docker buildx build --platform=linux/arm64 --platform=linux/arm/v7 --platform=linux/amd64 --tag itzg/minecraft-server:multiarch --push 
+```

Dockerfile

@@ -26,9 +26,9 @@ EXPOSE 25565 25575
 
 # hook into docker BuildKit --platform support
 # see https://docs.docker.com/engine/reference/builder/#automatic-platform-args-in-the-global-scope
-ARG TARGETOS=linux
-ARG TARGETARCH=arm
-ARG TARGETVARIANT=v7
+ARG TARGETOS
+ARG TARGETARCH
+ARG TARGETVARIANT
 
 ARG EASY_ADD_VER=0.7.1
 ADD https://github.com/itzg/easy-add/releases/download/${EASY_ADD_VER}/easy-add_${TARGETOS}_${TARGETARCH}${TARGETVARIANT} /usr/bin/easy-add