moby: ENV substitution problem with Windows builder

I’ve tried various combinations of $PATH and %PATH% and : and ;, but I just can’t seem to get ENV concatenation to work with Dockerfiles for Windows.

Other people seem to have given up too and just run RUN setx: https://github.com/StefanScherer/dockerfiles-windows/blob/197ac3a9a8fe8494915b9b62f517fa67151833cc/golang/Dockerfile#L9

setx /m may be the most correct workaround for now.

/cc @jhowardmsft @icecrime @dmp42

Output of docker info:

docker info
Containers: 71
 Running: 0
 Paused: 0
 Stopped: 71
Images: 45
Server Version: 1.11.0-dev
Storage Driver: windowsfilter
 Windows:
Logging Driver: json-file
Plugins:
 Volume: local
 Network: null nat
Kernel Version: 10.0 14316 (14316.1000.amd64fre.rs1_release.160402-2217)
Operating System: Windows Server 2016 Datacenter Technical Preview 5
OSType: windows
Architecture: x86_64
CPUs: 4
Total Memory: 1.242 GiB
Name: WIN-8F5EG07ROBB
ID: VKYM:E5F7:CMIT:4ZMS:OGUG:W43U:API4:ATWI:3S5X:RO54:DU4Q:YOFD
Docker Root Dir: C:\ProgramData\docker
Debug mode (client): false
Debug mode (server): true
 File Descriptors: -1
 Goroutines: 24
 System Time: 2016-04-13T18:46:29.3959003-07:00
 EventsListeners: 0
Username: friism4test
Registry: https://index.docker.io/v1/

Output of docker version:

docker version
Client:
 Version:      1.11.0-rc3
 API version:  1.23
 Go version:   go1.5.3
 Git commit:   eabf97a
 Built:        Fri Apr  1 23:33:49 2016
 OS/Arch:      windows/amd64

Server:
 Version:      1.11.0-dev
 API version:  1.24
 Go version:   go1.5.3
 Git commit:   577adcc
 Built:        Mon Apr 11 17:20:50 2016
 OS/Arch:      windows/amd64

Steps to reproduce the issue:

> "FROM windowsservercore`n RUN echo %PATH%`n ENV PATH c:\\go\\bin;`$PATH" | docker build -t test -
Sending build context to Docker daemon 2.048 kB
Step 1 : FROM windowsservercore
 ---> e1e5fb4dbc32
Step 2 : RUN echo %PATH%
 ---> Running in 91a3c7417bc3
C:\Windows\system32;C:\Windows;%localappdata%\Microsoft\WindowsApps;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\
 ---> 818d81426779
Removing intermediate container 91a3c7417bc3
Step 3 : ENV PATH c:\\go\\bin;$PATH
 ---> Running in f6ab92ac3110
 ---> d6b86fdef7e5
Removing intermediate container f6ab92ac3110
Successfully built d6b86fdef7e5
> docker run test cmd /c echo %PATH%
c:\go\bin;

Describe the results you received:

Incorrect PATH

Describe the results you expected:

Correctly concatenated PATH