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)
Add xz-utils to our deb-recommends since it's necessary for some images

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

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

Added a new documentation page explaining how to install Docker on
openSUSE.

Docker-DCO-1.1-Signed-off-by: Flavio Castelli <fcastelli@suse.com> (github: flavio)
Fix Travis to not check gofmt on vendor/*

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 #3586 from flavio/add_openSUSE_installation_docs

Added document explaining openSUSE installation
Merge pull request #3585 from tianon/remove-uid-ns-reference

Remove reference to nonexistent CONFIG_UID_NS
Merge pull request #3567 from tianon/dockerfile-add-symlink

Stop ADD from following symlinks outside the context when passed as the first argument
Merge pull request #3592 from tianon/fix-travis-vendor

Fix Travis to not check gofmt on vendor/*
Merge pull request #3581 from tianon/deb-xz-utils

Add xz-utils to our deb-recommends since it's necessary for some images