server/buildfile.go -> builder/builder.go; add maintainers file

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #7292 from erikh/buildfile_move

server/buildfile.go -> builder/builder.go
Bump libcontainer dep

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Add support for autodetected HOME from USER (if HOME is unset)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Remove pkg/user now that it's embedded in libcontainer

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)