Return error and not panic when cannot exec in dockerinit

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #3637 from crosbymichael/no-panic-on-exec

Return error and not panic when cannot exec in dockerinit
move inspect to 2 jobs

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
update attach to use the new job

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)