-w workdir flag is broken, tries to interpret argument in Windows

[sdwlig]

Please, check https://docs.docker.com/docker-for-win/troubleshoot/.
Issues without logs and details cannot be debugged, and will be closed.

• [x ] I have tried with the latest version of my channel (Stable or Edge)
• [x ] I have submitted Diagnostics
• [x ] I have included the Diagnostics ID in this Issue
• Windows Version: Win 10 pro
• Docker for Windows Version: Docker version 18.04.0-ce, build 3d479c0

Expected behavior

Working directory in container is set to the -w argument.

Actual behavior

Docker reports an error that the path is not absolute, showing a Windows absolute path.
docker run -w /tmp hello-world
C:\Program Files\Docker\Docker\Resources\bin\docker.exe: Error response from daemon: the working directory 'C:/Users/sdw/AppData/Local/Temp' is invalid, it needs to be an absolute path.
See 'C:\Program Files\Docker\Docker\Resources\bin\docker.exe run --help'.

Information

F759CEF6-89C7-45A4-97A3-F8A3D3503A68/2018-04-24_05-09-56
Works: docker run hello-world
Doesn't work: docker run -w /tmp hello-world

Steps to reproduce the behavior

1. Try to use the -w workdir flag which should be relative to the OS in the container, not the host.
2. ...

[sdwlig]

A similar error affects the command passed to docker:
Works: winpty docker exec -it abjs /../bin/bash
Broken: winpty docker exec -it abjs /bin/bash
OCI runtime exec failed: exec failed: container_linux.go:348: starting container process caused "exec: "C:/Program Files/Git/usr/bin/bash.exe": stat C:/Program Files/Git/usr/bi
n/bash.exe: no such file or directory": unknown

[mat007]

This is coming from Git for Windows, see their release notes (under Known issues):

If you specify command-line options starting with a slash, POSIX-to-Windows path conversion will kick in converting e.g. "/usr/bin/bash.exe" to "C:\Program Files\Git\usr\bin\bash.exe". When that is not desired -- e.g. "--upload-pack=/opt/git/bin/git-upload-pack" or "-L/regex/" -- you need to set the environment variable MSYS_NO_PATHCONV temporarily, like so:

MSYS_NO_PATHCONV=1 git blame -L/pathconv/ msys2_path_conv.cc

Alternatively, you can double the first slash to avoid POSIX-to-Windows path conversion, e.g. "//usr/bin/bash.exe".

In your case try docker run -w //tmp hello-world.

[sdwlig]

Thanks! I didn't dig deep enough to analyze MSYS...

I had tried // in some other cases. It appears that in that case I may have used a non-existent path which results in a similar error for a different reason.

The environment variable doesn't solve it completely, but with the // it seems to right now:
Works:
winpty docker exec -it nginx //bin/sh
Doesn't:
MSYS_NO_PATHCONV=1 winpty docker exec -it nginx /bin/sh
OCI runtime exec failed: exec failed: container_linux.go:348: starting container process caused "exec: "C:/Program Files/Git/usr/bin/sh.exe": stat C:/Program Files/Git/usr/bin/sh.exe: no such file or directory": unknown

[docker-robott]

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