docker: Refactor feat_set out of base layers.

Signed-off-by: Jason Volk <jason@zemos.net>

docker/Dockerfile.system

@@ -0,0 +1,31 @@
+# syntax = docker/dockerfile:1.11-labs
+
+ARG sys_name=debian
+ARG sys_version=testing-slim
+
+FROM ${sys_name}:${sys_version} AS system
+
+
+FROM input AS runtime
+ARG var_cache
+ARG var_lib_apt
+ARG packages
+ARG DEBIAN_FRONTEND
+
+WORKDIR /
+COPY --link --from=input . .
+
+ENV DEBIAN_FRONTEND="${DEBIAN_FRONTEND}"
+RUN \
+--mount=type=cache,dst=${var_cache},sharing=locked \
+--mount=type=cache,dst=${var_lib_apt},sharing=locked \
+<<EOF
+    echo $(uname -a) $0 $-
+    set -eux
+
+    keep_downloaded='Binary::apt::APT::Keep-Downloaded-Packages "true";'
+    echo "$keep_downloaded" > /etc/apt/apt.conf.d/keep-downloaded
+    rm -f /etc/apt/apt.conf.d/docker-clean
+
+    apt-get -y -U install --no-install-recommends ${packages}
+EOF