docs: document how to work with Linux kernel patches

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

docs/kernel-patches.md

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+# Working with Linux kernel patches for Moby
+
+We may apply patches to the Linux kernel used in Moby, primarily to
+cherry-pick some upstream patches or to add some additional
+functionality, not yet accepted upstream.  This document outlines the
+recommended procedure to handle these patches.
+
+Patches are located in `alpine/kernel/patches` and are maintained in
+`git am` format to keep important meta data such as the provenance of
+the patch.
+
+
+# Preparation
+
+Patches are applied to point releases of the linux stable tree. You need an up-to-date copy of that tree:
+```sh
+git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
+```
+
+Throughout we use the following variables:
+- `MOBYSRC`: Base directory of Moby Linux repository
+- `LINUXSRC`: Base directory of Linux stable kernel repository
+- `LINUXTAG`: Release tag to base the patches on
+e.g.:
+```sh
+MOBYSRC=~/src/docker/moby
+LINUXSRC=~/src/docker/linux-stable
+LINUXTAG=v4.4.24
+```
+
+# Updating the patches to a new kernel version
+
+Create a branch from a tag for the new patches, e.g.:
+```sh
+cd $LINUXSRC
+git branch ${LINUXTAG}-moby ${LINUXTAG}
+git checkout ${LINUXTAG}-moby
+```
+
+Import all the existing patches:
+```sh
+cd $LINUXSRC
+git am ${MOBYSRC}/alpine/kernel/patches/*.patch
+```
+
+If this causes merge conflicts resolve them as they arise and continue as instructed.
+
+Once finished, update the patches in Moby:
+```sh
+cd $LINUXSRC
+rm $MOBYSRC/alpine/kernel/patches
+git format-patch -o $MOBYSRC/alpine/kernel/patches ${LINUXTAG}..HEAD
+```
+
+Create a PR for Moby.
+
+
+# Adding new patches
+
+For patches from upstream Linux kernel versions, use cherry-picking:
+```sh
+git cherry-pick -x <sha of commit>
+```
+The `-x` ensures that the origin of the patch is recorded in the commit.
+
+
+For patches from the mailing list or patchworks, add a line like:
+```
+Origin: https://patchwork.ozlabs.org/patch/622404/
+```
+to the patch (after the `Signed-off-by` and `Cc` lines.
+
+
+For patches written from scratch, make sure it has a sensible commit
+messages as well as a DCO line.