No need to duplicate QEMU_VERSION build arg

Dockerfile

@@ -1,8 +1,6 @@
-ARG QEMU_VERSION=4.2.0
-
 # Build stage for qemu-system-arm
 FROM debian:stable-slim AS qemu-system-arm-builder
-ARG QEMU_VERSION
+ARG QEMU_VERSION=4.2.0
 ENV QEMU_TARBALL="qemu-${QEMU_VERSION}.tar.xz"
 WORKDIR /qemu
 
@@ -40,7 +38,6 @@ RUN strip "arm-softmmu/qemu-system-arm"
 # Build the dockerpi VM image
 FROM busybox:1.31 AS dockerpi-vm
 LABEL maintainer="Luke Childs <lukechilds123@gmail.com>"
-ARG QEMU_VERSION
 ARG RPI_KERNEL_URL="https://github.com/dhruvvyas90/qemu-rpi-kernel/archive/afe411f2c9b04730bcc6b2168cdc9adca224227c.zip"
 ARG RPI_KERNEL_CHECKSUM="295a22f1cd49ab51b9e7192103ee7c917624b063cc5ca2e11434164638aad5f4"