Docker build fails when passed context that is located inside Virtualbox shared folder

[pedropombeiro]

The upgrade from Docker 19.03.05 in https://app.vagrantup.com/StefanScherer/boxes/windows_2019_docker/versions/2020.07.17 caused this Docker bug to surface in the Vagrant boxes.

It manifests itself when running a docker build command which uses a context path that is located inside a Virtualbox shared mount:

unable to prepare context: unable to evaluate symlinks in context path: CreateFile \\vboxsvr\C:_GitLab-Runnerdockerfiles: The network name cannot be found.

In this case, the C:\GitLab-Runner symlink points to \\vboxsvr\C:_GitLab-Runner but Docker joins the path without a slash, so instead of \\vboxsvr\C:_GitLab-Runner\dockerfiles it sees \\vboxsvr\C:_GitLab-Runnerdockerfiles.

See https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/2555 and https://gitlab.com/gitlab-org/gitlab-runner/-/issues/27115 for more context.

[StefanScherer]

Do you know which box version you have used before the update?
It could also be related to VirtualBox 6 update. 🤔
I haven‘t used VirtualBox lately.
On which host OS do you encounter this problem?

[pedropombeiro]

Do you know which box version you have used before the update?

I'm not sure exactly which box version I was using before the update, however I went back through box versions and confirmed that it was introduced with 2020.07.17. If I use 2020.04.15 then docker build works fine.

It could also be related to VirtualBox 6 update. 🤔

I don't think it is related to a VirtualBox upgrade because we've tried downgrading to a previous version and the problem still manifested itself.

On which host OS do you encounter this problem?

On MacOS 10.15.7

[StefanScherer]

Thanks. I've tried my latest box with VirtualBox on my Mac (StefanScherer/windows_2019_docker (virtualbox, 2020.09.22) with Docker EE 19.03.12) and for a simple test shared volumes work fine. Ah, I see, a docker build doesn't work as it cannot
It works with a Docker CLI 19.03.8 (Go version: go1.12.17), but fails with newer versions (Go version: go1.13.13)

I did some more investigation.

Tried the nightly build from https://master.dockerproject.org/, also fails. It's also compiled with Go 1.13.x

Tried to build a Docker CLI 19.03.12 with Go 1.12.17 (I used https://github.com/StefanScherer/docker-cli-builder to build the Docker CLI), and this works.

So it's a change in underlying Golang. 🤔

PS C:\Users\stefanscherer\code\dockerfiles-windows\node\12\nano> C:\Users\stefanscherer\code\docker-cli-builder\docker.exe version
Client:
 Version:           19.03.12
 API version:       1.40
 Go version:        go1.12.17
 Git commit:        0ed913b8-
 Built:             11/11/2020 09:03:35
 OS/Arch:           windows/amd64
 Experimental:      false

Server: Docker Engine - Enterprise
 Engine:
  Version:          19.03.12
  API version:      1.40 (minimum version 1.24)
  Go version:       go1.13.13
  Git commit:       f295753ffd
  Built:            08/05/2020 19:26:41
  OS/Arch:          windows/amd64
  Experimental:     false

PS C:\Users\stefanscherer\code\dockerfiles-windows\node\12\nano> C:\Users\stefanscherer\code\docker-cli-builder\docker-19.03.12-go1.12.7.exe build -t test .
Sending build context to Docker daemon  791.3MB
Step 1/26 : ARG core=mcr.microsoft.com/windows/servercore:ltsc2019
Step 2/26 : ARG target=mcr.microsoft.com/windows/nanoserver:1809
...

I also tried latest Golang, but it also fails

PS C:\Users\stefanscherer\code\dockerfiles-windows\node\12\nano> C:\Users\stefanscherer\code\docker-cli-builder\docker-19.03.12-go1.15.4.exe build -t test .
unable to prepare context: unable to evaluate symlinks in context path: CreateFile \\vboxsvr\Users_stefanscherercode: The network name cannot be found.

[pedropombeiro]

Thanks for the thorough investigation @StefanScherer!