Add doc about Docker image.

doc/buildAndProgram.md

@@ -50,7 +50,7 @@ During the project generation, CMake created the following targets:
 - pinetime-mcuboot-app : build the firmware with the support of the bootloader (based on MCUBoot).
 - pinetime-graphics : small firmware that writes the boot graphics into the SPI flash.
 
-If you just want to build the project and run it on the Pinetime, using *pinetime-app* is recommanded. See ???? for more info about bootloader support.
+If you just want to build the project and run it on the Pinetime, using *pinetime-app* is recommanded. See [this page](../bootloader/README.md) for more info about bootloader support.
 
 Build:
 ```

doc/buildWithDocker.md

@@ -0,0 +1,33 @@
+# Build the project using Docker
+A [Docker image (Dockerfile)](../docker) containing all the build environment is available for X86_64 and AMD64 architectures. This image makes the build of the firmware and the generation of the DFU file for OTA.
+
+## Build the image
+The image is not (yet) available on DockerHub, you need to build it yourself, which is quite easy. The following commands must be run from the root of the project.
+
+If you are running on a x86_64 computer : 
+```
+docker image build -t infinitime-build docker/x86_64/
+```
+
+And if your are running on an ARM64 device (tested on RaspberryPi4 and Pine64 PineBookPro):
+```
+docker image build -t infinitime-build docker/arm64/
+```
+
+This operation will take some time. It builds a Docker image based on Ubuntu, install some packages, download the ARM toolchain, the NRF SDK, MCUBoot and adafruit-nrfutil.
+
+When this is done, a new image named *infinitime-build* is available.
+
+## Run a container to build the project:
+
+```
+docker run --rm -v <project_root>:/sources infinitime-build
+```
+
+Replace *<project_root>* by the path of the root of the project on your computer. For example:
+
+```
+docker run --rm -v /home/jf/git/PineTime:/sources infinitime-build
+```
+
+This will start a container, build the firmware and generate the MCUBoot image and the DFU file. The output of the build is stored in **<project_root>/built/output**.

docker/README.md

@@ -0,0 +1,2 @@
+Docker images and build script for building the project using Docker.
+See [this page for more info](../doc/buildWithDocker.md).