Add support for Overlay Volume Mounts when Podman is used with Docker Executor

This change adds support for mounting directories using the overlay file system. It introduces a small change to the parser to support the :O flag. This is useful for scenarios like speeding up container builds with minimal configuration.

As far as I know, Docker doesn’t support this option, but I think this is fine without additional checks.

Closes #31095 (closed), #29304 (closed).

Testing

• Build the gitlab-runner binary from this branch
• Configure a runner using the Podman executor with an overlay volume mount, e.g., volumes = ["/opt/overlay-dir:/opt/overlay-dir:O"]
• Create some sample file to the host machine at /opt/overlay-dir/file. E.g. mkdir -p /opt/overlay-dir && echo "hello from host" > /opt/overlay-dir/file

Use the following sample .gitlab-ci.yml:

image: docker.io/fedora:latest
stages:
  - test-overlay

test_podman_overlay:
  stage: test-overlay
  tags: ["overlay-test"]
  script:
    - cat /opt/overlay-dir/file
    - echo "hello from container" >> /opt/overlay-dir/file
    - cat /opt/overlay-dir/file

And observe that the setup actually works:

$ cat /opt/overlay-dir/file
hello from host
$ echo "hello from container" >> /opt/overlay-dir/file
$ cat /opt/overlay-dir/file
hello from host
hello from container

Successful sample pipeline: https://gitlab.com/napuu/runner-testing-project/-/jobs/9840010719. And sample using runner from the current main where this obviously doesn't work: https://gitlab.com/napuu/runner-testing-project/-/jobs/9840010390