Support for Dockerfile USER

[dtrudg]

Is your feature request related to a problem? Please describe. Some Docker containers were built with a Dockerfile containing a USER directive. The application was installed under the assumption that at runtime the specified user is active in the container. This is not the case with Singularity, so these containers may fail with permissions or application configuration errors.

Describe the solution you'd like ~~SingularityCE has a ‘fakeroot engine’ that is able to configure a container run so that subuid/subgid configuration is used. This type of functionality opens the possibility of carrying through USER specifications from Docker containers, so that their payload can run as the expected username.~~

Support for Dockerfile USER should be enabled by the ability to execute images through runc as a low-level engine, through 3.11 and 4.0.