This repository holds an opinionated centos (version 6) docker image to be used as a base docker image. Some inspiration sources are given at the end of this file.
Features:
- Updated image (at build time)
- Reasonable size (106MB compressed on the hub), reasonable number of layers (9) with a squashed Dockerfile (and another one to debug)
- Don't add too many packages by recompiling and installing just what's needed
- Don't break the upstream system :
- rebuild cronie et rsyslog7 from "upstream source RPM" (with some tuning to avoid too much dependencies in docker context)
- tricks to avoid distribution interferences (hidden virtualenv in non-standard prefix)
- Init system and multiple processes launcher/supervisor (S6)
- (optional) syslog daemon (rsyslog) and logger binary, can store locally or forward to another syslog with a simple environnement variable
- (optional) complete cron/anacron daemon (cronie)
- logrotate system
- Add epel yum repository but disabled by default (just add
--enablerepo=epelto your yum commands to use it) - Install envtpl generic tool to be able to automatically adjust configuration files from environnement
- we don't want a SSH daemon
Not really usefull (because it's mainly a base image to use in the FROM keyword) but you can play with it with (for example):
docker run -i -t thefab/centos-opinionated:latest bash
Available environnement variables:
DCO_CRONIE_START(if "1" (default) then starts the cron daemon)DCO_RSYSLOG_START(if "1" (default) then starts the syslog daemon)DCO_RSYSLOG_REMOTE_HOST(if "null" (default) then don't send logs to another syslog daemon, else forward logs to this hostname)DCO_RSYSLOG_REMOTE_PORT(the remote UDP port of the (optional) remote syslog daemon, default value: 514)DCO_RSYSLOG_LOCAL_FILES(if "1" (default) then log to local files in /var/log)
When you start this image with the default "entrypoint" (/init), it launch the S6 process supervisor (see links at the end)
provided by the s6-overlay.
First, it executes /etc/cont-init.d/* initialization (short) tasks. Then, it launchs /etc/services/*/run script. This script
must execute your daemon in a long-lived way and will have to deal with signals. If this "run script" exits, it will be automatically
restarted. An easy way to write a such script is to use the "exec" bash builtin with the "foreground mode" startup command of your service.
For example:
#!/bin/sh
exec /sbin/rsyslogd -nBecause of "exec", the script will be replaced by the launched command. So you won't have to deal with signals by yourself.
So a complete example to override this image with a new service on top of it can be:
$ find my_image
my_image/
my_image/Dockerfile
my_image/root
my_image/root/etc
my_image/root/etc/cont-init.d
my_image/root/etc/cont-init.d/my_initialization_script
my_image/root/etc/services.d
my_image/root/etc/services.d/myapp
my_image/root/etc/services.d/myapp/run
With my_image/Dockerfile like:
FROM thefab/centos-opinionated
COPY root /
And that's all ! Your custom service will be executed as well as rsyslog, cron services provides by the base image.
Yes, quotted from the s6-overlay README:
Using CMD is a really convenient way to take advantage of the s6-overlay. Your CMD can be given at build-time in the Dockerfile, or at runtime on the command line, either way is fine - it will be run under the s6 supervisor, and when it fails or exits, the container will exit. You can even run interactive programs under the s6 supervisor!
Please consult s6-overlay REAME for examples and more details.
Because of S6 overlay. If you want to use environnement variables in your scripts, you have to change your shebang to use the "with-contenv" S6 helper.
Example 1 (won't work):
#!/bin/bash
# don't work example
do_something_with ${MY_CONTAINER_ENV}Example 2 (will work):
#!/usr/bin/with-contenv bash
do_something_with ${MY_CONTAINER_ENV}This repository is only about CentOS 6 version.
A slightly different version of this repository for CentOS 7 is available at:
https://github.com/metwork-framework/docker-centos7-opinionated