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Description of problem:
The handling of stdout/stderr stream is not correct in tty mode (works fine in non-tty mode). You can see it on the example bellow. (`ls non_existing_file` prints into stderr stream)

Version-Release number of selected component (if applicable):
docker-1.1.1-1.el7.x86_64

How reproducible:
Always

Steps to Reproduce:
TTY:
[root@localhost ~]# docker run -t -a stderr fedora ls non_existing_file
[root@localhost ~]# docker run -t -a stdout fedora ls non_existing_file
ls: cannot access non_existing_file: No such file or directory
NON-TTY:
[root@localhost ~]# docker run -a stderr fedora ls non_existing_file
ls: cannot access non_existing_file: No such file or directory
[root@localhost ~]# docker run -a stdout fedora ls non_existing_file

Actual results:
In tty mode the output is visible only when STDOUT is attached, in non-tty mode it's visible in STDERR

Expected results:
In booth modes the output should be in STDERR stream.

Additional info:
When I use `ls /` it prints the info correctly into STDOUT in booth modes.

This looks like in -t mode it always prints stderr to stdout

Opened an issue https://github.com/docker/docker/issues/8040

Response is

"This is by-design. Seems like multiplexing can't be implemented for pty. There was some proposals about warnings when -t and -a stderr passed together."

So I guess I will close this as cantfix

Hey I'd love to see the warning when -a stderr is used with tty. If not would it be possible to have this in man pages?