azbarcea / base-docker

Custom docker base image.

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base-docker

Overview

This docker image is a custom base image.

It is intended to be as minimalistic as possible, while still:

  • Facilitating docker "entrypoint" best practices.
  • Assisting with debian package deployment.
  • Supporting organization-wide customizations.

Docker "entrypoint" Best Practices

Two instructions control what process is spawned when a docker container is created: ENTRYPOINT and CMD. The full documentation for these, and other instructions is available online, and it is strongly recommended that it is reviewed prior to creating Dockerfiles.

In brief, the ENTRYPOINT instruction specifies what process will be spawned. If it is defined in exec form the executable is launched directly; otherwise, it is provided as an argument to /bin/sh -c. The CMD instruction specifies how the process will be spawned. If it is defined in exec form it is appended directly to the ENTRYPOINT arguments; otherwise, it processed by a command shell before the resultant is appended.

By defining both instructions in the base image, and by embedding a shell script, the following is accomplished:

  • Dockerfile restrictions as to the existence and / or pairing of the instructions are satisfied.
  • A fail-safe command is present in every derived imaged.
  • Container initialization is facilitated as modifications to the embedded shell script will be executed before the main process.

NOTE: Although launching multiple processes using the entrypoint is possible, it is recommended that a process control system be used instead; such as supervisord.

Entrypoint Script

The emedded entrypoint script is located at /sbin/entrypoint and performs the following actions:

  1. All scripts in /etc/entrypoint.d/ are executed.
  2. The file $EP_RUN is created.
  3. The script arguments, from CMD are executed.

Environment Variables for Sub-scripts

  • EP_RUN - The fully-qualified path to the entrypoint run file: /var/local/container_initialized.
  • EP_USER - The name of the user as which to execute CMD.

Exported Functions for Sub-scripts

  • log - Logs to standard output.
  • render_template - Renders a given bash template.

Sample Entrypoint Script

#!/bin/bash
# /etc/entrypoint.d/foo

set -e

# Configure: foo
if [[ ! -e $EP_RUN ]] ; then
	log "Configuring $(basename $0) for first run ..."
	export VAR1=${VAR1:=VAL1}
	VAR2=VAL2 render_template /usr/local/share/foo.conf.template > /etc/foo/foo.conf
fi

Entrypoint Scripts

ca-certificates

The embedded entrypoint script is located at /etc/entrypoint.d/ca-certificates and performs the following actions:

  1. Certificates located in /usr/share/ca-certificates/docker/ are deployted into the containers trust store.

Debian Package Deployment

A typical Dockerfile will install packages similar to the following:

RUN apt-get update && \
	apt-get install --no-install-recommends --yes long list of packages && \
	apt-get clean && \
	apt-get autoremove && \
	rm --force --recursive /tmp/* /var/lib/apt/lists/* /var/tmp/*

While effective, this boilerplate code is on the edge of readability, and can be expressed more simply as RUN docker-apt <long list of packages>.

For that purpose, three scripts have been embedded in the base image:

  • docker-apt-install - Updates the aptitude repository list and installs given arguments.
  • docker-apt-clean - Cleans transient aptitude caches.
  • docker-apt - Invokes both of the above scripts.

Organization-wide Customizations

As of yet, this images does not contain any customizations; however, in the future it could contain common packages or initialization scripts. Typical uses could include:

  • Deployment of CA certificates.
  • Generation and / or distribution of SSH authorized_keys and known_hosts files.

Standard Configuration

Container Layout

/
├─ etc/
│  └─ entrypoint.d/
├─ sbin/
│  ├─ docker-apt
│  ├─ docker-apt-clean
│  ├─ docker-apt-install
│  ├─ entrypoint
│  └─ entrypoint.ca-certificates
└─ var/
   └─ local/
      └─ container_initialized

Exposed Ports

None.

Volumes

None.

Development

Source Control

About

Custom docker base image.

License:Apache License 2.0


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