add additional S3 related environment variables

Hi nextcloud/docker friends!

There's a few env vars that would be nice to set via the docker image. Per the docs here's the missing env vars:

• concurrency defaults to 5 [Note: This defines the maximum number of concurrent multipart uploads]

• proxy defaults to false

• timeout defaults to 15

• uploadPartSize defaults to 524288000

• putSizeLimit defaults to 104857600

• version defaults to latest

• verify_bucket_exists defaults to true [Note: Setting this to false after confirming the bucket has been created may provide a performance benefit, but may not be possible in multibucket scenarios.]

Each of the above would need a OBJECTSTORE_S3_ variable documented in the README.md, and also handled in .config/s3.config.php.

Thanks as always for your kind support over here!

• with love from nextcloud/helm :)

Review README.md and .config/s3.config.php, using the linked Nextcloud S3 documentation to confirm each requested setting and default. Document the seven OBJECTSTORE_S3_ variables in README.md and handle them in .config/s3.config.php; done means every listed setting can be configured through the image.