Windows Docker and paths

I use Docker Toolbox at home, as my laptop is running Windows Home and therefore cannot run Docker Desktop. Sometimes, mounting volumes can get tricky.

Let’s say that I’m working on a project and I want to try it out from within a Docker container. If I open up Docker Quickstart Terminal and run the following, everything works as expected (i.e. the current directory is mounted as /code inside my container):

docker run --rm -v $(pwd):/code -w /code -it perl bash

Most of the time though, I’ll open up my usual terminal or a terminal embedded in my IDE. In that case, the same command fails:

C:\Program Files\Docker Toolbox\docker.exe: Error response from daemon: the working
directory 'C:/Program Files/Git/code' is invalid, it needs to be an absolute path.
See 'C:\Program Files\Docker Toolbox\docker.exe run --help'.

Although I passed -w /code to set the working directory, Docker thinks I passed C:/Program Files/Git/code. If I remove the -w argument and keep just the volume, I get a similar error:

C:\Program Files\Docker Toolbox\docker.exe: Error response from daemon: invalid mode: \Program Files\Git\code.
See 'C:\Program Files\Docker Toolbox\docker.exe run --help'.

To overcome this, we need this variable:

MSYS_NO_PATHCONV=1

For example, this succeeds:

MSYS_NO_PATHCONV=1 docker run --rm -v $(pwd):/code -w /code -it perl bash

But why does it work from within Docker Quickstart Terminal? The answer lies in the script C:\Program Files\Docker Toolbox\start.sh. It defines docker as a function:

docker () {
  MSYS_NO_PATHCONV=1 docker.exe "$@"
}
export -f docker

which sets the environment variable and calls the Docker executable. By adding the same snippet in .bashrc, Docker behaves the same everywhere.