Fix docker based hooks failing on Windows - #1094

Fixes #1072

Docker for Windows

• mounts the volume passed via -v always using root:root as owner, even if a -u UID:GID option is passed
• newly created files (e.g. via touch) also use root:root as owner, even if the container is run as non-root user
• read/write access is also provided to non-root users, even if ls -l yields -rwxr-x-r-x as file permissions
• files created inside the container appear on the host as if they were created by the user running the docker command

This is because Docker for Windows currently implements host mounted volumes via SMB (see https://docs.docker.com/docker-for-windows/troubleshoot/#volumes), which handles the access to mounted files and does not allow changing permissions from inside the container (see docker/docs#3298).

Therefore the UID and GID we pass to docker can be chosen arbitrarily, because the owner of files on the host will match the Windows user running docker and not the user in the docker container.

There are also some other caveats when it comes to running linux docker images via Docker for Windows, e.g. you have to be careful about line endings (moby/moby#24388).

With these changes I was able to successfully run a hook on Windows using language: docker_image for a personal project of mine, therefore I would like to see something like this getting merged.

What are your thoughts about this?