Stop ADD from following symlinks outside the context when passed as the first argument

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Fix "foo: no such file or directory" test failure, and normalize creation of custom error to always depend on if os.IsNotExist(err) so we don't hide other errors that might crop up in these tests

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3567 from tianon/dockerfile-add-symlink

Stop ADD from following symlinks outside the context when passed as the first argument