Creating a container in order to execute jobs on other containers

This is an example of how to create a docker container to run scheduled commands using the crontab on other containers.

If you just need a container to run jobs that do not need to interact with additional containers, I recommend you to have a look at my other repository docker-cron-example.

Also known as DooD (Docker outside of Docker), I have implemented this by the past when playing with continuous integration systems that used docker to build applications based on pull-requests. The basic idea of DooD is to access the host’s Docker installation from within the container. Basically, we link the docker socket (/var/run/docker.sock) from the host to the cron container using the -v flag.

You can build the image using the following commands:

$ docker build -t docker-cron-ooc-example .

You can run the container like this:

$ docker run -it \
-v /var/run/docker.sock:/var/run/docker.sock:ro \
--name my-docker-cron-ooc \
docker-cron-ooc-example

Few minutes after having started the container you should see something similar to this:

crond: crond (busybox 1.24.2) started, log level 8
crond: USER root pid  12 cmd docker ps >> /var/log/cron/cron.log 2>&1
CONTAINER ID        IMAGE                     COMMAND             CREATED             STATUS              PORTS               NAMES
161557771d32        docker-cron-ooc-example   "/run-crond.sh"     9 seconds ago       Up 8 seconds                            my-docker-cron-ooc
crond: USER root pid  19 cmd docker ps >> /var/log/cron/cron.log 2>&1
CONTAINER ID        IMAGE                     COMMAND             CREATED              STATUS              PORTS               NAMES
161557771d32        docker-cron-ooc-example   "/run-crond.sh"     About a minute ago   Up About a minute                       my-docker-cron-ooc
crond: USER root pid  27 cmd docker ps >> /var/log/cron/cron.log 2>&1
CONTAINER ID        IMAGE                     COMMAND             CREATED             STATUS              PORTS               NAMES
161557771d32        docker-cron-ooc-example   "/run-crond.sh"     2 minutes ago       Up 2 minutes                            my-docker-cron-ooc
crond: USER root pid  33 cmd docker ps >> /var/log/cron/cron.log 2>&1
CONTAINER ID        IMAGE                     COMMAND             CREATED             STATUS              PORTS               NAMES
161557771d32        docker-cron-ooc-example   "/run-crond.sh"     3 minutes ago       Up 3 minutes                            my-docker-cron-ooc

Using environment variables

The following command included in run-crond.sh loads all the environment variables, gets the one starting with ENV_, combine them at the top of the /tmp/crontab file and finally move the result to /etc/cron.d/crontab

env | egrep '^ENV_' | cat - /tmp/crontab > /etc/cron.d/crontab

Add environment variables to the docker run command using the -e flag like this. You should be able to use them in your script.

$ docker run -it -e ENV_NAME=f00 docker-cron-ooc-example

Accessing to the logs

You can access to the crontab output by running docker logs on your container

$ docker logs <container_name>

The Dockerfile

FROM gliderlabs/alpine:3.5
MAINTAINER "Julien Dubreuil"
RUN apk update && apk add --no-cache docker (see the Notes section)

COPY crontab /tmp/crontab

COPY run-crond.sh /run-crond.sh
RUN chmod -v +x /run-crond.sh

RUN mkdir -p /var/log/cron && touch /var/log/cron/cron.log

CMD ["/run-crond.sh"]

1. The FROM command specify our base image. It used a alpine on steroids image as our base image.
2. Next, we copy the crontab file to the /tmp directory of the container in order to make it available as declared in the run-crond.sh file.
3. Then, we copy the crontab run script to the container and make it executable.
4. We Create a default log file for the cron job.
5. Finally, we specify the command that container will execute on startup.

Notes

Normally you can get rid of the docker installation inside the cron container by linking the docker binary file from the host to the container. Unfortunately after several tries I haven't been able to achieve it on my mac (maybe a mac issue...). Using a volume you should be able to add the docker binary like this.

-v $(which docker):/usr/bin/docker

Copyright and License

MIT License, see LICENSE for details. Copyright (c) 2017 Julien Dubreuil

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