Allow developers to build docker with debuginfo included

If you execute

DEBUG=-g hack/make.sh dynbinary

Docker will be build with the debug info making it easier to use
cgdb or lightide to debug.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge branch 'master' of github.com:docker/docker into debug

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge pull request #9262 from rhatdan/debug

Allow developers to build docker with debuginfo included