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 <[email protected]>                               
                                                                                 
# 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 

$ docker build

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:

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

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

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

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:

The rest of the Dockerfile does the following:

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.