Docker: Combining Multiple Images

Docker, a containerization platform, enables the isolation and packaging of applications with their dependencies. While it's commonly used to manage individual images, there may be scenarios where you need to combine multiple images into a single unit.

Combining Generic and Specific Images: A Scenario

Consider a scenario where you have generic Java and MySQL images, and you want to create a single image that combines both Java and MySQL. This can be achieved using a modified approach that involves DockerMake, an open-source tool that manages image inheritance.

Using DockerMake for Image Combination

DockerMake employs a YAML file to outline the composition of the combined image. The DockerMake.yml file describes the inheritance hierarchy and the build steps for each component image. Here's an example DockerMake.yml file that combines genericA, genericB, and customBase images into the specificAB image:

specificAB:
  requires:
    - genericA
    - genericB

genericA:
  requires:
     - customBase
  build_directory: [some local directory]
  build: |
    # Add Dockerfile commands here (e.g., ADD, RUN)

genericB:
  requires:
    - customBase
  build: |
    # Additional Dockerfile commands (e.g., apt-get, ENV)

customBase:
  FROM: debian:jessie
  build: |
    # Base image setup commands (e.g., apt-get update)

Building the Combined Image

To build the combined image using DockerMake, follow these steps:

1. Install DockerMake using pip: pip install dockermake.
2. Create a DockerMake.yml file following the inheritance hierarchy.
3. Run the DockerMake build command: docker-make specificAB

This process generates the necessary Dockerfiles based on the DockerMake.yml file and builds the combined image. The resulting image, in this case specificAB, will possess the functionalities of both genericA and genericB images, providing a single unit with the desired application stack.