Instead of building the actual image, `build_config` will serialize a subset of
dockerfile ast into *runconfig.Config
Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Upstream-commit: 7f091eca704970a5cb8a4fdab295dae543c47a89
Component: engine