dont close proxy's stdout/stderr prematurely

Fix #8428

When stdout/stderr is closed prematurely, the proxy's writes to stdout/stderr
(i.e. `log.Errorf/log.Printf`) will returns with EPIPE error, and go runtime
will terminate the proxy when stdout/stderr writes trigger 10 EPIPE errors.

This fixes stdout/stderr by continuing to drain their output if the proxy
successfully starts. Stdout/Stderr will be closed when `proxy.Wait` returns.

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
Shouldn't we check that here we have something like `docker-proxy -proto tcp -host-ip 0.0.0.0 -host-port 12345 -container-ip 172.17.0.7 -container-port 80`?