fix: Extract a template for the nextcloud container (app/cronjob)

[MartinKirchner]

Description of the change

Use a helper template to define the containers for app/cronjob.

Benefits

• The two containers stay in sync.

Possible drawbacks

None known.

Applicable issues

Additional information

Checklist

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