Too open permissions for mounted secrets

[Cryptophobia]

Hi. After a little dig into the Workflow I discovered that every application container that is running on deis/slugrunner has an object store credentials volume attached to it. I know that a slugrunner need an access to S3 storage to download a slug tarball, but shouldn't this be considered as a security issue that every user on this container (including application itself) has a read access to those files?

We thought that we could use a defaultMode option in Kubernetes that restrict permissions for mounted volumes to a root user, but it seems that both the init and execution processes of the slugrunner are running as user slug.

Related to: deis/slugrunner#58