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)
I don't like this goroutines, I think this can be solved by `log.SetOutput(ioutil.Discard)` in `execProxy`. Because proxy uses only `log` package to output info.