feat: add an environment var to skip the find-and-chown

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Addresses #172

I don't know whether this is acceptable at all, but we are creating our base docker images for Plone that do not do this find-and-chown thing, because we run Plone with ZEO as storage, and when you have a log of blobs it takes a lot of time to start.

That's why I decided to add an environment var, that if set, avoids doing the find-and-chown thing.

I have added the documentation in the documentation repo: plone/documentation#2035

[davisagli]

I think the main reason we have this command here at all is because when a new volume is mounted in Docker, it starts out owned by the host user rather than the container's plone user. So, this is a bit of a hack to make sure a new site can be created with the correct permissions after a new volume is mounted.

With that in mind: what if we changed it to only do the recursive find-and-chown if the top-level /data folders are not already owned by plone. Then it should still solve the problem with initializing the volume, but it won't re-run it on every startup.

I don't have a big problem with the environment variable, but:

• it seems like we could make this smart enough that it isn't necessary
• if we do add it, it should be documented (oh, I see there's a linked PR for that)