fix(cronjob): change sidecar to cronstable for nonroot compatible (#733)

[antoinetran]

Description of the change

In shared Kubernetes environment which disable root containers, the cron as sidecar cannot work because cron implementation from alpine requires root. This change will replace to cronstable which is docker compliant and does not need root to work.

Benefits

Can work in restricted Kubernetes environment:

• with non root policy
• without PV as readwrite many, so impossible to use cronjob as Kubernetes cronjob

Possible drawbacks

Adds another image, but small footprint (~40MiB for alpine flavor)

Applicable issues

• fixes nextcloud-cron does not work in OpenShift #733

Additional information

Checklist

• I have read the CONTRIBUTING.md doc.
• DCO has been signed off on the commit.
• Chart version bumped in Chart.yaml according to semver.
• (optional) Parameters are documented in the README.md