Updating Path multiple times in the same RUN command

Question

Consider the following Dockerfile. On the last lines, first git is installed, and then something is appended to the path environment variable.

FROM mcr.microsoft.com/windows/servercore:ltsc2022

SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop';"]

RUN Invoke-Expression ((New-Object System.Net.WebClient).DownloadString('https://chocolatey.org/install.ps1'))

RUN choco install -y git
RUN [Environment]::SetEnvironmentVariable('Path', $Env:Path + ';C:\my-path', [EnvironmentVariableTarget]::Machine)

After the build completes, the path looks like this, so git was added to the path.

C:\ProgramData\chocolatey\bin;C:\Program Files\Git\cmd;C:\my-path;

Here is an equivalent Dockerfile, but I have made the last lines into a single RUN command for optimization.

FROM mcr.microsoft.com/windows/servercore:ltsc2022

SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop';"]

RUN Invoke-Expression ((New-Object System.Net.WebClient).DownloadString('https://chocolatey.org/install.ps1'))

RUN choco install -y git; \
    [Environment]::SetEnvironmentVariable('Path', $Env:Path + ';C:\my-path', [EnvironmentVariableTarget]::Machine)

After the build completes, git is not on the path!

C:\ProgramData\chocolatey\bin;C:\my-path;

Why is this, and how can I fix it?

I'm not familiar with PowerShell, but in standard Linux containers and the Bourne shell, environment variable changes in a `RUN` command are lost at the end of that command, and you need to use `ENV` to change the environment. See for example [In a Dockerfile, How to update PATH environment variable?](https://stackoverflow.com/questions/27093612/in-a-dockerfile-how-to-update-path-environment-variable). — David Maze, May 12 '22 at 13:02
@DavidMaze Good suggestion, but doesn't seem to work. `ENV FOO="bar"` works, but `ENV PATH="bar"` messes something up. An error is issued when using the `path` later. — Magnar Myrtveit, May 12 '22 at 13:23
ENV does not work in Windows containers: https://github.com/moby/moby/issues/30033 — Magnar Myrtveit, May 13 '22 at 07:45

score 0 · Accepted Answer · answered May 13 '22 at 07:50

A workaround is to use cmd instead of powershell.

Both the following approaches work:

RUN choco install -y git; \
    cmd /c "setx path '%path%;C:\my-path'"

SHELL ["cmd", "/S", "/C"]
RUN choco install -y git && \
    setx path "%path%;C:\my-path"
SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop';"]

Updating Path multiple times in the same RUN command

1 Answers1