Docker for Windows building added prefix `/var/lib/docker/tmp/` for COPY?

When you run the build command

docker build .

The . in that command indicates the "build context" that gets sent to the (possibly remote) docker engine. All of the COPY and ADD commands must be from inside this directory (or from a previous stage with multi stage builds). You can also exclude files from this context using the .dockerignore file. The tmp directory is a uniquely generated internal directory of docker's consisting of this build context.

To fix your issue, you need to make sure ProcessFiles/ProcessFiles.csproj exists in the directory where you run your build from, and that you didn't exclude it from the context with the .dockerignore file.

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Edit: based on your comments, change your copy command to:

COPY ProcessFiles.csproj ProcessFiles/