kernel: Add a Dockerfile to make it easier to configure kernels

The new Dockerfile.kconfig can be used, via the 'kconfig' make target
to build a 'linuxkit/kconfig' images. This images contains the patched
source and default kernel configs for all supported kernels.

It's useful to updating the kernel config files.

While at it, also update the alpine base.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

kernel/Dockerfile

@@ -1,4 +1,4 @@
-FROM linuxkit/alpine:87a0cd10449d72f374f950004467737dbf440630 AS kernel-build
+FROM linuxkit/alpine:a120ad6aead3fe583eaa20e9b75a05ac1b3487da AS kernel-build
 RUN apk add \
     argp-standalone \
     automake \

kernel/Dockerfile.kconfig

@@ -0,0 +1,36 @@
+FROM linuxkit/alpine:a120ad6aead3fe583eaa20e9b75a05ac1b3487da AS kernel-build
+RUN apk add \
+    argp-standalone \
+    build-base \
+    curl \
+    diffutils \
+    ncurses-dev \
+    tar \
+    xz
+
+ARG KERNEL_VERSIONS
+
+# There is no simple way to copy directories with wild cards as needed
+# for patches-*. Copy the entire dir instead.
+COPY / /
+
+# Unpack kernels (download if not present)
+RUN for VERSION in ${KERNEL_VERSIONS}; do \
+        KERNEL_SOURCE=https://www.kernel.org/pub/linux/kernel/v4.x/linux-${VERSION}.tar.xz && \
+        [ -f sources/linux-${VERSION}.tar.xz ] || curl -fSLo sources/linux-${VERSION}.tar.xz ${KERNEL_SOURCE} && \
+        tar xf sources/linux-${VERSION}.tar.xz; \
+    done
+
+# Apply patches to all kernels and move config files into place
+RUN for VERSION in ${KERNEL_VERSIONS}; do \
+        SERIES=${VERSION%.*}.x && \
+        cd /linux-${VERSION} && \
+        for patch in /patches-${SERIES}/*.patch; do \
+            echo "Applying $patch" && \
+            patch -p1 < "$patch"; \
+        done && \
+        mv /kernel_config-${SERIES}-x86_64 arch/x86/configs/x86_64_defconfig && \
+        mv /kernel_config-${SERIES}-aarch64 arch/arm64/configs/defconfig; \
+    done
+
+ENTRYPOINT ["/bin/sh"]

kernel/Makefile

@@ -49,6 +49,8 @@ COMMIT_LABEL=--label org.opencontainers.image.revision=$(REPO_COMMIT)
 endif
 LABELS=$(REPO_LABEL) $(COMMIT_LABEL)
 
+KERNEL_VERSIONS=
+
 .PHONY: check tag push
 # Targets:
 # fetch: Downloads the kernel sources into ./sources
@@ -76,6 +78,7 @@ define kernel
 ifeq ($(3),)
 sources/linux-$(1).tar.xz: Makefile | sources
 	curl -fsSLo sources/linux-$(1).tar.xz https://www.kernel.org/pub/linux/kernel/v4.x/linux-$(1).tar.xz 
+KERNEL_VERSIONS+=$(1)
 endif
 
 build_$(2)$(3): Dockerfile Makefile $(wildcard patches-$(2)/*) $(wildcard kernel_config-$(2)*) kernel_config.debug | sources
@@ -133,3 +136,9 @@ endef
 $(eval $(call kernel,4.9.44,4.9.x))
 $(eval $(call kernel,4.9.44,4.9.x,_dbg))
 $(eval $(call kernel,4.4.83,4.4.x))
+
+# Target for kernel config
+kconfig: | sources
+	docker build --no-cache -f Dockerfile.kconfig \
+		--build-arg KERNEL_VERSIONS="$(KERNEL_VERSIONS)" \
+		-t linuxkit/kconfig  .