Start a system service in the Postgres docker container - linux

Question

I want to extends the postgres:10.2 Dockerfile in order to add a cron job doing some SQL queries at specific dates:
FROM postgres:10.2
COPY task-purge.sh /usr/local/share/
RUN chown postgres:postgres /usr/local/share/task-purge.sh
RUN chmod 700 /usr/local/share/task-purge.sh
COPY query-task-purge.sql /usr/local/share/
RUN chown postgres:postgres /usr/local/share/query-task-purge.sql
RUN chmod 700 /usr/local/share/query-task-purge.sql
The problem is: the cron service is not started:
Inside the docker container:
root#5c17ce88c333:/# service cron status
[FAIL] cron is not running ... failed!
root#5c17ce88c333:/# pgrep cron
root#5c17ce88c333:/#
I have difficulties to start it ...
In the Dockerfile, I tried :
To add RUN service cron start: nothing change
To add CMD service cron start: when the container starts, it ends with Starting periodic command scheduler: cron without starting the DB.
To add CMD postgres && service cron start: when the container starts, it ends with "root" execution of the PostgreSQL server is not permitted. without starting the DB.
To add a wrappere CMD script like https://docs.docker.com/config/containers/multi-service_container/: same behaviour.
To add ENTRYPOINT "docker-entrypoint.sh" && service cron start: idem
To add service cron start in a new docker-entrypoint.sh (modified from the official postgres:10.2 Dockerfile https://hub.docker.com/layers/postgres/library/postgres/10.2/images/sha256-4b6b7bd361a3b7b69531b2c16766a38b0f3a89e9243f5a49ff16180dd2d42273?context=explore): Starting periodic command scheduler: croncron: can't open or create /var/run/crond.pid: Permission denied failed!
To add update-rc.d cron defaults && update-rc.d cron enable to the docker-entrypoint.sh: nothing change.
To add set -- su-exec root:root /bin/bash -c "service cron start": nothing change
To add set -- su-exec root:root /bin/bash -c "update-rc.d cron defaults && update-rc.d cron enable": nothing change
To add gosu root:root /bin/bash -c "service cron start": the container ends with error: failed switching to "root:root": operation not permitted.
To add exec gosu root:root /bin/bash -c "service cron start": the container ends with Starting periodic command scheduler: cron.
Do you have any idea how I can run a system service before postgres start ? And I want to extends postgres:10.2.
Thanks !

Answer 1

Ok, I have understand why ... I answer my own question to help everybody : the docker-entrypoint.sh script runs a exec gosu postgres "$BASH_SOURCE" "$#" command (even in the last postgres version: https://github.com/docker-library/postgres/blob/master/docker-entrypoint.sh) which call again this script but as postgres user.
So every system operations needs to be executed before this command.
For example: write a function called system_configure and call it before that line:
# ... (outside main)
system_configure() {
echo "[x] Crontab service start ..."
service cron start
echo "[x] Crontab service started"
}
# ... (inside main function path)
system_configure
exec gosu postgres "$BASH_SOURCE" "$#"
# ... (end main)
You could also use any other docker image like ubuntu with supervisord, but prefers distroless image for security reasons.

Answer 2

Just run it once in your Dockerfile using:
RUN service cron start