Rationale

I have a use case where Girder is deployed to manage a large dataset stored on a filesystem that can be only read. Similarly to how S3 assetstore handles it, it should be possible to indicate that assetstore is readOnly and allow to assestore/:id/import files from it.

Steps to reproduce

1. Login as admin. Navigate to Admin console / Assetstores / Create new Filesystem assetstore
2. Provide a name and path to readonly directory, e.g. /root/

Expected result

Provide a read only checkbox and allow to create the assetstore.

Actual result

Unable to write into directory "/root/".