Create scratch image to contain kernel

Author: timower

Dockerfile

@@ -3,10 +3,13 @@ ARG toltec_image=ghcr.io/toltec-dev/base:v3.1
 ARG rm2_stuff_tag=v0.1.2
 ARG fw_version=3.5.2.1807
 ARG linux_release=5.8.18
+
+# By default use a cached linux kernel. To build locally pass:
+#  --build-arg linux_image=linux-build
 ARG linux_image=ghcr.io/timower/rm-docker-linux:linux-image
 
 # Step 1: Build Linux for the emulator
-FROM $toltec_image as linux-build
+FROM $toltec_image as linux-builder
 
 RUN apt-get update && \
     apt-get install -y bison bc lzop libssl-dev flex
@@ -34,6 +37,10 @@ RUN make O=imx7 ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -j $(nproc) && \
     cp imx7/arch/arm/boot/dts/imx7d-rm.dtb /opt && \
     rm -rf imx7
 
+# This container just needs to kernel and device tree
+FROM scratch AS linux-build
+COPY --from=linux-builder /opt/zImage /opt/imx7d-rm.dtb /opt
+
 # Dummy stage to use in the arg below
 FROM $linux_image AS linux-image