When you pipe to docker build on Windows it fails under Conemu.
It is probably related to these lines which explicitly target Conemu.

Versions

C:\> docker version
Client:
 Version:      1.12.3
 API version:  1.24
 Go version:   go1.6.3
 Git commit:   6b644ec
 Built:        Thu Oct 27 00:09:21 2016
 OS/Arch:      windows/amd64
 Experimental: true

Server:
 Version:      1.12.3
 API version:  1.24
 Go version:   go1.6.3
 Git commit:   6b644ec
 Built:        Thu Oct 27 00:09:21 2016
 OS/Arch:      linux/amd64
 Experimental: true

ConEmu build: x64
ConEmu version: 16.10.22.0
OS version: Windows 10 x64 (Version 1607 OS Build 14393.447)
Used shell version (Far Manager, git-bash, cmd, powershell, cygwin, whatever): ???

Problem description

When using Docker for Windows, while using Linux containers, if you run on Conemu:

And PowerShell:

 echo 'FROM hello-world' | docker build -t temp -

or

'FROM hello-world' | docker build -t temp -

Or on cmd:

echo FROM hello-world | docker build -t temp -

You get this error:

unable to prepare context: failed to peek context header from STDIN: Incorrect function.

The command works on cmd and PowerShell without Conemu.