Permission error when repository content shall be written to disk

I’m trying to use an image from Docker Hub, makarius/isabelle, for automated builds. However, if usage of this image is enabled, checking out my repository fails with the error EACCES: permission denied, open '/__w/⟨repository-name⟩/⟨repository-name⟩/⟨uuid⟩.tar.gz'. The repository is downloaded using the GitHub REST API.

A minimal workflow file that triggers this error is as follows:

name: Automated checkout

on: [push]

jobs:
  checkout:
    runs-on: ubuntu-latest
    container: makarius/isabelle
    steps:
      - name: Check out repository
        uses: actions/checkout@v2

Unfortunately, the above error message, while mentioning a path, does not tell, on which machine the respective directories should exist and what permissions they should have.

From the documentation, I cannot tell, what actions/checkout@v2 is trying to achieve. Does the virtual host try to write the data into the container and doesn’t have write permissions? Or does the container try to copy the data from the virtual host onto its own disk, and, if yes, is the problem that it cannot read the data or that it cannot write the data?

Are there any constraints a docker image to be used with actions/checkout@v2 has to fulfill? If yes, where are these documented?

Start by reproducing the minimal workflow with actions/checkout@v2 and the makarius/isabelle container, then trace where the GitHub REST API download is written. Check the checkout action's container assumptions and permission handling. Done means the permission cause and required container constraints are identified and either documented or corrected.