feat: only do a full chown of the /data directory if requested or if the /data directory is not owned by plone

[erral]

• I signed and returned the Plone Contributor Agreement, and received and accepted an invitation to join a team in the Plone GitHub organization.
• I verified there aren't any other open pull requests for the same change.
• I followed the guidelines in Contributing to Plone.
• I successfully ran code quality checks on my changes locally.
• I successfully ran tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes.
• I included a change log entry in my commits.

---

As an alternative to #198, here I check that the data folder is owned by plone and run the find-and-chown only if it is not.

I also add an option to explicitly run the the find-and-chown if a given env var is passed.

[erral]

I have added an output message to signal that the docker image is running the find & chown commands.

I have also added some tests for this functionality.

If accepted, I will forward-port this to the 6.2 branch too.