Container Tips

Dockerfile tips

Pass release information to the container build

Docker allows you to pass build-time arguments with the --build-arg <varname>=<value> flag. These are referenced in the Dockerfile using the ARG instruction.

You can use these to write release information to the image so it is retrievable during runtime. Then it is easy to determine exactly what code you are looking at in a given environment.

Converting Docker build arguments into environment variables and writing the info to the image filesystem.

ARG RELEASE
ARG APP_NAME
ARG INSTANCE_NAME
ARG BRANCH_NAME
ENV RELEASE=${RELEASE:-FORGOT_BUILD_ARG_RELEASE} \
    APP_NAME=${APP_NAME:-FORGOT_BUILD_ARG_APP_NAME} \
    INSTANCE_NAME=${INSTANCE_NAME:-FORGOT_BUILD_ARG_INSTANCE_NAME} \
    BRANCH_NAME=${BRANCH_NAME:-FORGOT_BUILD_ARG_BRANCH_NAME}
RUN echo "{\"APP_NAME\": \"$APP_NAME\", \"INSTANCE_NAME\": \"$INSTANCE_NAME\", \"RELEASE\": \"$RELEASE\"}" > /pub/RELEASE.txt

Entrypoint tips

Use echo to help debugging

When you are trying to figure out why something is not working correctly when you run your Docker image, all you have is the logged output. Be generous with some echo statements to give information about what is going on.

Using echo to indicate which branch of an if statement was taken. It is clear if the logs show Starting Gunicorn that the DEBUG environment variable was not True.

if [ "$DEBUG" == "True" ]; then
    echo "DEBUG MODE: Starting Waitress"
    waitress-serve --unix-socket=$HOMEDIR/gunicorn.sock --expose-tracebacks conf.docker_wsgi:application
else
    echo "Starting Gunicorn"
    gunicorn --preload --config $GUNICORNCONF conf.docker_wsgi:application
fi

Using echo to show an unrecognized value. Both the unrecognized value and the fact it was unrecognized will show in the logs.

case "$SITE" in
    en)
        echo "Using default English configuration"
        ;;
    es)
        echo "Copying over Spanish nGinx configuration"
        cp $HOMEDIR/sites/es/conf/nginx-core.conf /etc/nginx/nginx.conf
        ;;
    de)
        echo "Copying over German nGinx configuration"
        cp $HOMEDIR/sites/de/conf/nginx-core.conf /etc/nginx/nginx.conf
        ;;
    fr)
        echo "Copying over French nGinx configuration"
        cp $HOMEDIR/sites/fr/conf/nginx-core.conf /etc/nginx/nginx.conf
        ;;
    *)
        echo "I don't recognize SITE=$SITE"
        ;;
esac

Detect overridden commands

In order to do this properly, you must use the exec form of ENTRYPOINT in your Dockerfile. This means your ENTRYPOINT looks like:

ENTRYPOINT ["executable", "param1", "param2"]

Command line arguments to docker run <image> will be appended after all elements in an exec form ENTRYPOINT, and will override all elements specified using CMD.

With that in place, you can use the $# built-in bash variable to see if additional command-line arguments were passed.

If $# is not 0, extra command-line arguments were passed.

if [ $# -gt 0 ]; then
    echo "Running overridden command $@"
    exec $@
else
    echo "Normal operation"
    # Do whatever that is
fi