Using Docker as a Yocto build system container

Embedded systems often have a long product lifetime so software needs to be built with the same Yocto version over several years in a predictable way. Older versions of Yocto often have problems in running with state-of-the-art distributions.

To work around this, there are several alternatives:

• Keep a build machine with a fixed operating system. This is problematic as the machine also ages and it may suffer from hardware problems and need re-installation.
• Use a cloud machine with a fixed operating system. Not everyone has this type of infrastructure available and it usually has a price tag attached.
• Build in a virtual machine such as VMware or VirtualBox. This affects the build performance significantly.
• Use a Docker Yocto builder container. This has the advantage of providing the same isolation as the virtual machine but with a much better build performance.

We saw how to run a docker container in the Using the Toaster web interface recipe. Now we will see how to create our own Docker image to use as a Yocto builder.

Getting ready

Docker is able to build images automatically by reading instructions from a text file called a Dockerfile. Dockerfiles can be layered on top of each other, so to create a Docker Yocto builder image we would start by using a Ubuntu 16.04 Docker image or one of the other supported distributions, and sequentially configure the image.

An example Dockerfile for a Yocto builder follows:

FROM ubuntu:16.04                                                                
MAINTAINER Alex Gonzalez <[e-mail]>                               
                                                                                 
# Upgrade system and Yocto Proyect basic dependencies                            
RUN apt-get update && apt-get -y upgrade && apt-get -y install gawk wget git-core diffstat unzip texinfo gcc-multilib build-essential chrpath socat cpio python python3 python3-pip python3-pexpect xz-utils debianutils iputils-ping libsdl1.2-dev xterm curl 
                                                                                 
# Set up locales                                                                 
RUN apt-get -y install locales apt-utils sudo && dpkg-reconfigure locales && locale-gen en_US.UTF-8 && update-locale LC_ALL=en_US.UTF-8 LANG=en_US.UTF-8 
ENV LANG en_US.utf8                                                              
                                                                                 
# Clean up APT when done.                                                        
RUN apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*               
                                                                                 
# Replace dash with bash                                                         
RUN rm /bin/sh && ln -s bash /bin/sh                                        
                                                                                 
# User management                                                                
RUN groupadd -g 1000 build && useradd -u 1000 -g 1000 -ms /bin/bash build && usermod -a -G sudo build && usermod -a -G users build 
                                                                                 
# Install repo                                                                   
RUN curl -o /usr/local/bin/repo https://storage.googleapis.com/git-repo-downloads/repo && chmod a+x /usr/local/bin/repo 
                                                                                 
# Run as build user from the installation path                                   
ENV YOCTO_INSTALL_PATH "/opt/yocto"                                              
RUN install -o 1000 -g 1000 -d $YOCTO_INSTALL_PATH                               
USER build                                                                       
WORKDIR ${YOCTO_INSTALL_PATH}                                                    
                                                                                 
# Set the Yocto release                                                          
ENV YOCTO_RELEASE "rocko"                                                         
                                                                                 
# Install Poky                                                                   
RUN git clone --branch ${YOCTO_RELEASE} git://git.yoctoproject.org/poky          
                                                                                 
# Install FSL community BSP                                                      
RUN mkdir -p ${YOCTO_INSTALL_PATH}/fsl-community-bsp && cd ${YOCTO_INSTALL_PATH}/fsl-community-bsp && repo init -u https://github.com/Freescale/fsl-community-bsp-platform -b ${YOCTO_RELEASE} && repo sync 
                                                                                 
# Create a build directory for the FSL community BSP                             
RUN mkdir -p ${YOCTO_INSTALL_PATH}/fsl-community-bsp/build                       
                                                                                 
# Make /home/build the working directory                                         
WORKDIR /home/build 

How to do it...

1. To build the container locally from the directory containing the Dockerfile, run the following command:

$ docker build

2. However, there is no need to build it locally as the container is automatically built on the Docker registry: https://hub.docker.com/r/yoctocookbook2ndedition/docker-yocto-builder

First create an empty folder owned by a user with the same uid and gid that the build user inside the container:

$ sudo install -o 1000 -g 1000 -d /opt/yocto/docker-yocto-builder

And change inside the new directory:

$ cd /opt/yocto/docker-yocto-builder

To run the container and map its /home/build folder to the current directory, type:

$ docker run -it --rm -v $PWD:/home/build yoctocookbook2ndedition/docker-yocto-builder  

Where:

• • -it instructs Docker to keep stdin open even when the container is not attached and assign a pseudo-tty to the interactive shell
  • --rm instructs Docker to remove the container on exit
  • -v maps the host current directory as the /home/build container volume

3. We can now instruct the container to build a Poky project with:

build@container$ source /opt/yocto/poky/oe-init-build-env qemuarm build@container$ MACHINE=qemuarm bitbake core-image-minimal

4. To build a FSL community BSP project, you need to map the /opt/yocto/fsl-community-bsp/build container directory with the current directory as the setup-environment script only works when the build directory is under the installation folder:

$ docker run -it --rm -v $PWD:/opt/yocto/fsl-community-bsp/build yoctocookbook2ndedition/docker-yocto-builder  

5. Then we can run the following command inside the container to create a new project and start a build:

build@container$ cd /opt/yocto/fsl-community-bsp/
build@container$ mkdir -p wandboard
build@container$ MACHINE=wandboard DISTRO=poky source setup-environment build
build@container$ bitbake core-image-minimal

How it works...

Instructing Docker to start the image creation process with a Ubuntu 16.04 image is as easy as starting the Dockerfile with the following:

FROM ubuntu:16.04

To inherit a Docker image, you use the Dockerfile FROM syntax.

Other commands used in the Dockerfile are:

• RUN, which will run the specified command in a new layer and commit the result
• ENV, to set an environmental variable
• USER, which sets the username to use for RUN and CMD instructions following it
• WORKDIR, which sets the working directory for RUN and CMD instructions that follow it
• CMD, which provides the default executable for the container, in this case the Bash shell

The rest of the Dockerfile does the following:

• Updates Ubuntu 16.04 to the latest packages
• Installs Yocto dependencies
• Sets up the locale for the container
• Adds a new build user
• Installs both Poky and the FSL community BSP release

The image has Poky installed at /opt/yocto/poky and the FSL community BSP installed at /opt/yocto/fsl-community-bsp. When it starts, the default directory is/home/build.

The usual way to work with a docker container is to instruct it to run commands but store the output in the host filesystem.

In our case, we instruct the container to run BitBake for us, but we map the build directories to the host by doing the external volume mapping when the container is initialized. In that way, all the build output is stored on the host machine.

See also

• Docker documentation for the image builder can be found at https://docs.docker.com/engine/reference/builder/