Use a base image for building kernel

As we do not have a simple way to hash Debian, use the Docker
sha256 until we switch to Alpine.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

alpine/base/debian-build-kernel/Dockerfile

@@ -0,0 +1,24 @@
+FROM debian:jessie
+
+RUN apt-get update && apt-get -y upgrade && apt-get -y install \
+  unzip \
+  xz-utils \
+  curl \
+  bc \
+  build-essential \
+  cpio \
+  gcc libc6 libc6-dev \
+  kmod \
+  squashfs-tools \
+  genisoimage \
+  xorriso \
+  syslinux \
+  isolinux \
+  automake \
+  pkg-config \
+  git \
+  ncurses-dev \
+  p7zip-full \
+  lzop \
+  wget \
+  vim

alpine/base/debian-build-kernel/Makefile

@@ -0,0 +1,10 @@
+# no easy hashing scheme for Debian, will switch to Alpine soon so temporarily use sha256 
+.PHONY: push
+
+BASE=debian:jessie
+IMAGE=debian-build-kernel
+
+push:
+	docker pull $(BASE)
+	tar cf - Dockerfile | docker build --no-cache -t mobylinux/$(IMAGE):latest -
+	docker push mobylinux/$(IMAGE):latest

alpine/kernel/Dockerfile

@@ -1,33 +1,10 @@
-FROM debian:jessie
+FROM mobylinux/debian-build-kernel@sha256:412d36481df6123efc9a5a51656e13164f8178d11d682c63de8de82d43abd0d2
 
 ARG KERNEL_VERSION=4.4.20
 ARG DEBUG=0
 
 ENV KERNEL_SOURCE=https://www.kernel.org/pub/linux/kernel/v4.x/linux-${KERNEL_VERSION}.tar.xz
 
-RUN apt-get update && apt-get -y upgrade && apt-get -y install \
-  unzip \
-  xz-utils \
-  curl \
-  bc \
-  build-essential \
-  cpio \
-  gcc libc6 libc6-dev \
-  kmod \
-  squashfs-tools \
-  genisoimage \
-  xorriso \
-  syslinux \
-  isolinux \
-  automake \
-  pkg-config \
-  git \
-  ncurses-dev \
-  p7zip-full \
-  lzop \
-  wget \
-  vim
-
 RUN curl -sSL -o linux-${KERNEL_VERSION}.tar.xz ${KERNEL_SOURCE}
 
 RUN cat linux-${KERNEL_VERSION}.tar.xz | tar --absolute-names -xJ &&  mv /linux-${KERNEL_VERSION} /linux
@@ -37,11 +14,13 @@ ENV AUFS_REPO       https://github.com/sfjro/aufs4-standalone
 ENV AUFS_BRANCH     aufs4.4
 ENV AUFS_COMMIT     8be74e42b108f9cbbb0d59a87468505a48ff2757
 
-# Download AUFS and patch kernel
+# Download AUFS
 RUN git clone -b "$AUFS_BRANCH" "$AUFS_REPO" /aufs && \
     cd /aufs && \
-    git checkout -q "$AUFS_COMMIT" && \
-    cd /linux && \
+    git checkout -q "$AUFS_COMMIT"
+
+# patch kernel with aufs
+RUN cd /linux && \
     cp -r /aufs/Documentation /linux && \
     cp -r /aufs/fs /linux && \
     cp -r /aufs/include/uapi/linux/aufs_type.h /linux/include/uapi/linux/ && \