Creating the Base Image

Introduction

The base image is the foundation for all other images.

This image should include common packages and configuration used for all other images including:

  • Builder images
  • Development images
  • Production (Release) images

Note

It is quite possible (and probably recommended to reduce complexity) that a base image may span mutiples projects. Always strive for reusability where possible.

Building the Base Image

To build the base image, use the make image command as follows:

make image docker/base

The docker/base argument is the relative path to the folder where the base image Dockerfile is located. Under the hood the make image docker/base command is calling:

docker build -t cloudhotspot/sampledjangoapp-base:latest -f docker/base/Dockerfile docker/base

The naming of the base folder is important as the Makefile uses this as a convention to set attach a build context to the base image name.

For example, assuming the following environment configuration:

  • REPO_NS = cloudhotspot
  • IMAGE_NAME = sampledjangoapp
  • VERSION = latest

Running make image docker/base will create an image tagged as follows:

  • cloudhotspot/sampledjangoapp-base:latest

If your base image Dockerfile was located in docker/foundation, then make image docker/foundation would create the following image:

  • cloudhotspot/sampledjangoapp-foundation:latest

Base Image Internals

The sample base image in the PyPackage repository is located in the docker/base folder and includes the following files:

  • Dockerfile
  • entrypoint.sh

Dockerfile

The sample Dockerfile for the base image is listed below:

FROM ubuntu:trusty
MAINTAINER Justin Menga <justin.menga@cloudhotspot.co>

# Set mirrors to NZ and install common packages
RUN sed -i "s/http:\/\/archive./http:\/\/nz.archive./g" /etc/apt/sources.list && \
    apt-get update && \
    apt-get install -qyy -o APT::Install-Recommends=false -o APT::Install-Suggests=false python-virtualenv python libffi6 openssl libpython2.7

# Create virtual environment
RUN virtualenv /appenv

# Upgrade PIP
RUN . /appenv/bin/activate; pip install pip==7.1.2

# Activate the virtual environment
ADD entrypoint.sh /usr/local/bin/entrypoint.sh
RUN chmod +x /usr/local/bin/entrypoint.sh
ENTRYPOINT ["entrypoint.sh"]

As you can see, the base image is based from the official Ubuntu Trusty Docker image and includes the following packages:

  • python (Python 2.7)
  • python-virtualenv
  • openssl
  • libpython2.7
  • libffi6

The base image also:

  • Creates a virtual environment
  • Installs a recent version of PIP in the virtual environment
  • Adds the entrypoint.sh shell script as the entrypoint for the image (discussed below)

entrypoint.sh

The entrypoint.sh shell script is very simple:

#!/bin/bash
. /appenv/bin/activate
exec $@

All this script does is simply activate the virtual environment and then execute any arguments passed to the script. This leverages the behaviour of Docker entrypoints, where any arguments passed at the end of the docker run command are passed as arguments to the image entrypoint.

This allows arbitrary commands to be run within the virtual environment, increasing the utility of the image. For example, the following executes the command pip -V in the Docker container to display the installed pip version:

$ docker run -it --rm cloudhotspot/sampledjangoapp-base pip -V
pip 7.1.2 from /appenv/local/lib/python2.7/site-packages (python 2.7)

Note

Using exec ensures the command executed will run as PID 1 in the Docker container (rather than the bash shell). This is important for ensuring the container can exit and shutdown cleanly.