Disable msys mangling in sgx-lkl-docker, when detected

[bengreenier]

Hi there - first off, thanks for a great solution for TEE development. This project is great! 🎉

In using this locally, I've found a few issues when running using docker+win10 in non-WSL bash environments (specific msys, via git-for-windows) This PR resolves those issues, by controlling how msys-like environments (and only these) handle path mangling, and informing them to simply not do this. With mangling disabled, paths resolve correctly, and msys doesn't mess with things unnecessarily.

For me, these few lines are the difference between this project working out of the box, and requiring some fiddling, so I thought I'd try and see if you folks were open to landing a quality of life fix like this. 🙏

Background Details 📝

By default, these msys environments try to help the user, by converting things like /bin/bash to C:/Program Files/Git/usr/bin/bash. It's quite smart about when to do this, but you can see it occur with the following example:

$ docker run -it ubuntu:18.04 /bin/bash
C:\Program Files\Docker\Docker\Resources\bin\docker.exe: Error response from daemon: OCI runtime create failed: container_linux.go:345: starting container process caused "exec: \"C:/Program Files/Git/usr/bin/bash\": stat C:/Program Files/Git/usr/bin/bash: no such file or directory": unknown.

This is because docker expands to docker.exe (as we don't have docker in msys, only in the windows host itself) and msys detects this difference, and tries to help. It assumes that if we're calling an exe and giving a path to something that is also technically an exe (in this case, /bin/bash, since it knows that locally /bin/bash is actually C:/Program Files/Git/usr/bin/bash.exe) we want to pass the path to the exe, rather than the actual given literal path. So it applies this transformation for us, and when docker runs it gets the exe path, not the /bin/bash literal.

This fails to execute inside the docker environment, as there is no C:/Program Files/Git/usr/bin/bash.exe, there is only /bin/bash.

One workaround is to replace all instances of /bin/bash with //bin/bash, which prevents this msys behavior and should not cause issues in most linux distros it seems, as that first / is ignored. This however, would be a much more significant change.

The proposed workaround here, is to simply set the flag that msys uses to determine if it should execute this behavior such that it's informed not to. This looks like MSYS_NO_PATHCONV=1.

[cpriebe]

Thanks a lot! That looks good to me.