Enable crond in an Alpine container - docker

Question

Context
I'm trying to schedule some ingestion jobs in an Alpine container. It took me a while to understand why my cron jobs did not start: crond doesn't seems to be running
rc-service -l | grep crond
According to Alpine's documentation, crond must first be started with openrc (i.e. some kind of systemctl). Here is the Dockerfile
FROM python:3.7-alpine
# set work directory
WORKDIR /usr/src/collector
RUN apk update \
&& apk add curl openrc
# ======>>>> HERE !!!!!
RUN rc-service crond start && rc-update add crond
# install dependencies
RUN pip install --upgrade pip
RUN pip install pipenv
COPY ./Pipfile /usr/src/collector/Pipfile
RUN pipenv install --skip-lock --system --dev
# copy entrypoint.sh
COPY ./entrypoint.sh /usr/src/collector/entrypoint.sh
# copy project
COPY . /usr/src/collector/
# run entrypoint.sh
ENTRYPOINT ["/usr/src/collector/entrypoint.sh"]
entrypoint.sh merely appends the jobs at the end of /etc/crontabs/root
Problem
I'm getting the following error:
* rc-service: service `crond' does not exist
ERROR: Service 'collector' failed to build: The command '/bin/sh -c rc-service crond start && rc-update add crond' returned a non-zero code: 1
Things are starting to feel a bit circular. How can rc-service not recognizing a service while, in the same time:
sh seems to know the name crond,
there was a /etc/crontabs/root
What am I missing?

Answer 1

Some Alpine Docker containers are missing the busybox-initscripts package. Just append that to the end of your apk add command, and crond should run as a service.
You might also need to remove the following line from your Dockerfile since it seems as if busybox-initscripts runs crond as a service immediately after installation:
RUN rc-service crond start && rc-update add crond

Answer 2

I was able to fix this by adding the crond command to the docker-entrypoint.sh, prior to the actual script commands.
e.g.:
#!/bin/sh
set -e
crond &
...(rest of the original script)
This way the crond is reloaded as a detached process.
So all the steps needed was to
Find and copy the entrypoint.sh to the build folder. I did this from a running container:
docker cp <running container name>:<path to script>/entrypoint.sh <path to Dockerfile folder>/entrypoint.sh
Modify the entrypoint.sh as stated above
Include the entrypoint.sh again in the Dockerfile used for building the custom image. Dockerfile example:
...
COPY docker-entrypoint.sh <path to script>/entrypoint.sh
RUN chmod +x <path to script>/entrypoint.sh
...
And then just build and use the new custom image:
docker build -t <registry if used>/<image name>:<tag> .

Answer 3

Have faced the issue either. My solution for containers is to execute crond inside of screen
# apk add screen --no-cache
# screen -dmS crond crond -f -l 0

Answer 4

FROM alpine:latest
RUN touch crontab.tmp \
&& echo '* * * * * echo "123"' > crontab.tmp \
&& crontab crontab.tmp \
&& rm -rf crontab.tmp
CMD ["/usr/sbin/crond", "-f", "-d", "0"]
ref: https://gist.github.com/mhubig/a01276e17496e9fd6648cf426d9ceeec

Answer 5

Run this one: apk add openrc --no-cache