ContainerExecAttach implicitly runs ContainerExecStart while attaching
to stdout/stderr.
Ref: e02bc91dcb/client/container_exec.go (L40)
Calling both can lead to a race condition as observed in #627
Fixes: #627
Co-authored-by: Markus Wolf <markus.wolf@new-work.se>
Co-authored-by: Markus Wolf <markus.wolf@new-work.se>