initial docker support

It builds for both RPi3 and BBB, but there is a mount
warning for BBB, which may lead to it not working properly.

Changelog: Docker environment for running mender-convert

Signed-off-by: Eystein Måløy Stenberg <eystein.maloy.stenberg@northern.tech>

docker/Dockerfile

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+FROM ubuntu:18.04
+
+ARG MENDER_ARTIFACT_VERSION=2.3.0
+ARG MENDER_CONVERT_VERSION=1.0.0
+
+RUN apt-get update && apt-get install -y \
+    kpartx \
+    bison \
+    flex \
+    mtools \
+    parted \
+    mtd-utils \
+    e2fsprogs \
+    u-boot-tools \
+    pigz \
+    device-tree-compiler \
+    autoconf \
+    autotools-dev \
+    libtool \
+    pkg-config \
+    python \
+# for mender-convert to run (mkfs.vfat is required for boot partition)
+    sudo \
+    dosfstools \
+# to compile U-Boot
+    bc \
+# to download gcc toolchain and mender-artifact
+    wget \
+# to extract gcc toolchain
+    xz-utils \
+# to download mender-convert and U-Boot sources
+    git
+
+# Disable sanity checks made by mtools. These checks reject copy/paste operations on converted disk images.
+RUN echo "mtools_skip_check=1" >> $HOME/.mtoolsrc
+
+# Needed while we use older U-Boot version for Raspberry Pi
+# https://tracker.mender.io/browse/MEN-2198
+# Assumes $(pwd) is /
+RUN wget -nc -q http://releases.linaro.org/components/toolchain/binaries/6.3-2017.05/arm-linux-gnueabihf/gcc-linaro-6.3.1-2017.05-x86_64_arm-linux-gnueabihf.tar.xz \
+    && tar -xJf gcc-linaro-6.3.1-2017.05-x86_64_arm-linux-gnueabihf.tar.xz \
+    && rm gcc-linaro-6.3.1-2017.05-x86_64_arm-linux-gnueabihf.tar.xz \
+    && echo export PATH=$PATH:/gcc-linaro-6.3.1-2017.05-x86_64_arm-linux-gnueabihf/bin >> /root/.bashrc
+
+
+RUN wget -q -O /usr/bin/mender-artifact https://d1b0l86ne08fsf.cloudfront.net/mender-artifact/$MENDER_ARTIFACT_VERSION/mender-artifact \
+    && chmod +x /usr/bin/mender-artifact
+
+
+# TODO: support selecting tag of mender-convert with MENDER_CONVERT_VERSION
+# TODO: consider lighter way to download to avoid git dependency
+RUN git clone https://github.com/mendersoftware/mender-convert.git

docker/README.md

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+Docker environment for mender-convert
+=====================================
+
+In order to correctly set up partitions and bootloaders, mender-convert has many dependencies,
+and their version and name vary between Linux distributions.
+
+To make using mender-convert easier, a reference setup using a Ubuntu 18.04 Docker container
+is provided.
+
+You need to [install Docker Engine CE](https://docs.docker.com/install) to use this environment.
+
+
+## Build the mender-convert container image
+
+To build a container based on Ubuntu 18.04 with all required dependencies for mender-convert,
+copy this directory to your workstation and change the current directory to it.
+
+Then run
+
+```bash
+./build.sh
+```
+
+This will create a container image you can use to run mender-convert.
+
+
+## Use the mender-convert container image
+
+Create a directory `input` under the directory where you copied these files (`build.sh`, `run.sh`, etc.):
+
+```bash
+mkdir input
+```
+
+Put your raw disk image into `input`, e.g. `input/2018-10-09-raspbian-stretch.img`.
+Then put a Mender client binary compiled for your device into `input/mender-1.6.0-arm-linux-gnueabihf-gcc`.
+See [the Mender documentation on how to cross-compile the Mender client for your device](https://docs.mender.io/development/client-configuration/cross-compiling).
+
+After you have built or pulled the mender-convert container image,
+you can run it with
+
+```bash
+./run.sh
+```
+
+This will drop you into a shell environment (interactive mode).
+Change to the mender-convert directory:
+
+```bash
+cd /mender-convert
+```
+
+You can now run mender-convert with your desired options, e.g.
+
+```bash
+DEVICE_TYPE="raspberrypi3"
+RAW_DISK_IMAGE="input/2018-10-09-raspbian-stretch.img"
+MENDER_CLIENT="input/mender-1.6.0-arm-linux-gnueabihf-gcc"
+
+ARTIFACT_NAME="2018-10-09-raspbian-stretch"
+MENDER_DISK_IMAGE="2018-10-09-raspbian-stretch.sdimg"
+TENANT_TOKEN="<INSERT-TOKEN-FROM Hosted Mender>"
+
+./mender-convert from-raw-disk-image                        \
+            --raw-disk-image $RAW_DISK_IMAGE                \
+            --mender-disk-image $MENDER_DISK_IMAGE          \
+            --device-type $DEVICE_TYPE                      \
+            --mender-client $MENDER_CLIENT                  \
+            --artifact-name $ARTIFACT_NAME                  \
+            --bootloader-toolchain arm-linux-gnueabihf      \
+            --server-url "https://hosted.mender.io"         \
+            --tenant-token $TENANT_TOKEN
+```
+
+Conversion will take 10-15 minutes, depending on your storage and resources available.
+You can watch `output/build.log` for progress and diagnostics information.
+
+After it finishes, you can find your images in the `output` directory on your host machine!

docker/build.sh

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+#!/bin/sh
+
+set -e
+
+IMAGE_NAME=mender-convert
+
+docker build . -t $IMAGE_NAME

docker/run.sh

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+#!/bin/sh
+
+set -e
+
+IMAGE_NAME=mender-convert
+
+INPUT_DIR="$(pwd)/input"
+OUTPUT_DIR="$(pwd)/output"
+
+mkdir -p $OUTPUT_DIR
+
+docker run -it \
+       --mount type=bind,source=$INPUT_DIR,target=/mender-convert/input,readonly \
+       --mount type=bind,source=$OUTPUT_DIR,target=/mender-convert/output \
+       --privileged=true \
+       $IMAGE_NAME