Introduce a new agent flag and support docker volume chown to task user.

Currently, docker volume is always mounted as root, which is not accessible by non-root task users. For security concerns, there are use cases that operator may only allow non-root users to run as container user and docker volume needs to be supported for those non-root users.

A new agent flag is needed to make this support configurable, because chown-ing a docker volume may be limited to some use case - e.g., multiple non-root users on different hosts sharing the same docker volume simultaneously. Operators are expected to turn on this flag if their cluster's docker volume is not shared by multiple non-root users.