Run the job.container as the same user of the host VM by default

Description

Run the containers as the same user running on the host (the VM) by default, or at least provide an option. The related docker option would be docker run --user "$(id -u):$(id -g)".

Containers job.container runs as the container's default user by default, which often is root. This causes some weird behaviors because the workspace on the GitHub runner is mounted to the container, and no other user (other than the Runner user) has permission to write on it. This is what makes @actions/checkout fail when running on containers.

As Jenkins does, this should be made by default. Users who might want to run as a different user can still use the --user docker option.

Workaround

As suggested by @xanantis:

jobs:
  configure:
    runs-on: ubuntu-latest
    outputs:
      uid_gid: ${{ steps.get-user.outputs.uid_gid }}
    steps:
      - id: get-user
        run: echo "::set-output name=uid_gid::$(id -u):$(id -g)"

  clone-and-install:
    needs: configure
    runs-on: ubuntu-latest
    container:
      image: mcr.microsoft.com/vscode/devcontainers/base:ubuntu
      options: --user ${{ needs.configure.outputs.uid_gid }}
    steps:
      - uses: actions/checkout@v2

It's even funny to have to do such a thing.

The issue names no source file or test; begin by locating the runner code that assembles job.container Docker options. Compare the current default-user behavior with the documented --user workaround, and consider how an explicit user option remains available. Done means containers default to the host VM user's UID/GID without breaking custom --user settings.