Resolve "optionally enable nextcloud's debug mode"

Thank you for making a Merge Request. Before you merge this, please make sure that you:

• Updated chart version number
• Manually tested chart changes with the openappstack bootstrap script
• Created an MR in openappstack/openappstack that applies the changes from this MR (openappstack!136)
• Tests in that MR in openappstack/openappstack succeed

Closes #15 (closed)