python3-slim (buildroot)

common.cfg

# caching
BR2_DL_DIR="/cache/dl"
BR2_CCACHE=y
BR2_CCACHE_DIR="/cache/ccache"
BR2_CCACHE_USE_BASEDIR=y

# arch
BR2_arm=y

# toolchains
BR2_TOOLCHAIN_BUILDROOT_MUSL=y
BR2_TOOLCHAIN_BUILDROOT_CXX=y
BR2_KERNEL_HEADERS_4_19=y
BR2_GCC_ENABLE_LTO=y

# init
BR2_INIT_NONE=y

# system utils
BR2_PACKAGE_BUSYBOX=y
BR2_SYSTEM_BIN_SH_NONE=n
BR2_PACKAGE_IFUPDOWN_SCRIPTS=n

BR2_PACKAGE_PYTHON3=y

Dockerfile

#syntax=docker/dockerfile:1.2

FROM klutchell/buildroot-base:2020.11 as build

# copy common config
COPY common.cfg ./.config

# hadolint ignore=SC2215
RUN --mount=type=cache,target=/cache,uid=1000,gid=1000,sharing=private \
    make olddefconfig && make source && make

# hadolint ignore=DL3002
USER root

WORKDIR /rootfs

# extract the rootfs so we can just COPY in future layers
RUN tar xpf /home/br-user/output/images/rootfs.tar -C /rootfs

FROM scratch

COPY --from=build rootfs/ /

CMD [ "/bin/busybox", "sh" ]