passing stdin to `az container exec` fails with ENOTTY

## Describe the bug
passing stdin to `az container exec` fails with ENOTTY (`termios.error: (25, 'Inappropriate ioctl for device')`)

**Command Name**
`az container exec`

**Errors:**
```
(25, 'Inappropriate ioctl for device')
Traceback (most recent call last):
python3.8/site-packages/knack/cli.py, ln 215, in invoke
cmd_result = self.invocation.execute(args)
cli/core/commands/__init__.py, ln 654, in execute
raise ex
cli/core/commands/__init__.py, ln 718, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
cli/core/commands/__init__.py, ln 711, in _run_job
six.reraise(*sys.exc_info())
...
cli/command_modules/container/custom.py, ln 623, in container_exec
_start_exec_pipe(execContainerResponse.web_socket_uri, execContainerResponse.password)
cli/command_modules/container/custom.py, ln 657, in _start_exec_pipe
oldtty = termios.tcgetattr(sys.stdin)
termios.error: (25, 'Inappropriate ioctl for device')
```

## To Reproduce:

`echo "echo foo" | az container exec -g $MY_RESOURCE_GROUP --name $MY_DEBIAN_CONTAINER --exec-command "/bin/bash"`

## Expected Behavior

The stdin content should be passed to the process, and "foo" should be printed.

## Environment Summary
```
macOS-10.15.6-x86_64-i386-64bit
Python 3.8.5
Installer: HOMEBREW

azure-cli 2.11.1

```
## Additional Context
https://github.com/Azure/azure-cli/issues/13352 ("az container exec in azure devops throws (25, 'Inappropriate ioctl for device')") has the same error code but reproduction steps are different.