docker: document image creation process

Create docker/README.md, which documents the use of the docker/
folder. This will make it easier to find documentation on how the docker
images are built. This guide was originally found on the docker hub page
of savoirfairelinux/ring-ubuntu32 and was not under source control.

Change-Id: I8b8ab6179b6e1ba36974437a16463821f42f2a84
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>

docker/README.md

+# Packaging dockerfiles
+
+This folder contains Dockerfiles for building Ring. They are used by
+Makefile.packaging, at the root of this repository
+
+## 32bit images
+
+Some of the Dockerfiles refer to 32bit images in the savoirfairelinux docker
+hub organization. These images are generated with the following method:
+
+
+### Debian/Ubuntu
+
+- Download 32bit system iso or image
+- Install the system in a container or virtual magine
+- Run the following commands from the system:
+    * `apt update`
+    * `apt upgrade`
+    * `apt install git docker.io debootstrap`
+    * `git clone https://github.com/moby/moby.git`
+    * `./moby/contrib/mkimage.sh -t savoirfairelinux/<distro>32:<distroversion> debootstrap --variant=minbase --arch=i386 <codename>`
+    * `docker login`
+    * `docker push savoirfairelinux/<distro>32:<distroversion>`
+
+### Fedora
+
+TODO