KodeKloud Notes

In Docker, every custom image starts “FROM” another image—its parent. Tracing this chain leads to the special scratch image, the true base for all builds.

Note

A base image is the origin of an image chain (often scratch), while a parent image is the one directly referenced by your FROM instruction.

1. Custom Web Application Image

Create a minimal Apache-based web server by extending the official HTTPD image:

FROM httpd
COPY index.html /usr/local/apache2/htdocs/index.html

Parent image: httpd
Your COPY command layers application files on top of the Apache HTTPD server.

2. Inside the HTTPD Official Image

The official HTTPD image itself builds on Debian:

FROM debian:buster-slim

ENV HTTPD_PREFIX=/usr/local/apache2
ENV PATH=$HTTPD_PREFIX/bin:$PATH
WORKDIR $HTTPD_PREFIX

# Installation steps for Apache HTTPD...

Parent image: debian:buster-slim
Installs Apache HTTPD on a minimal Debian base.

3. Exploring the Debian Base Image

The Debian “slim” image originates from scratch:

FROM scratch
ADD rootfs.tar.xz /
CMD ["bash"]

Base image: scratch
Unpacks a minimal Debian filesystem from a tarball.

Warning

You cannot push or pull the scratch image—it's only referenced in FROM scratch directives.

Table: Image Lineage at a Glance

Layer	Image Tag	Parent	Description
Application	`custom-web-app:latest`	`httpd:latest`	Apache HTTPD plus your `index.html`
HTTPD Official	`httpd:latest`	`debian:buster-slim`	Official Apache HTTPD on Debian
Debian Base	`debian:buster-slim`	`scratch`	Minimal Debian filesystem from `scratch`

4. Additional Official Image Chains

Ubuntu → MongoDB

Ubuntu minimal from scratch:

FROM scratch
ADD ubuntu-xenial-core-cloudimg-amd64-root.tar.gz /

MongoDB on Ubuntu:

FROM ubuntu:xenial

RUN groupadd -r mongodb \
 && useradd -r -g mongodb mongodb

RUN set -eux; \
    apt-get update; \
    apt-get install -y --no-install-recommends \
        ca-certificates jq numactl; \
    if ! command -v ps >/dev/null; then \
        apt-get install -y --no-install-recommends procps; \
    fi

WordPress → PHP → Debian

The official WordPress image builds on the official PHP image, which in turn uses Debian—demonstrating another scratch → Debian → PHP → WordPress chain.

The image illustrates a comparison between base and parent images, showing different software stacks built on top of base images like Debian and Ubuntu, leading to applications like WordPress, MongoDB, and a custom web app.

5. The `scratch` Image: Docker’s True Base

scratch is Docker’s reserved, empty image.
It marks the very beginning of any build.
By adding a minimal OS filesystem on top of scratch, maintainers create base images (Debian, Alpine, Ubuntu), which then serve as parents for application and service images.

For more details on building and using base images, see the official Docker documentation on base images.

Links and References

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