document how to use an oauth token with container image

Following the docs in the README, I want to use the container image and the temporary (oauth) tokens.

The README describes how to run the auth sub-command, but it assumes a local Golang binary, rather than a container image.

Here is how I ran it with podman, including z for SELinux support:

mkdir -p ~/.config/gitlab-mcp/
podman run -it --rm  -v ~/.config/gitlab-mcp:/.config/gitlab-mcp:z registry.gitlab.com/fforster/gitlab-mcp:latest auth```

I can walkthrough the flow and it gives me Success, and then I have a token outside the container in my homedir at ~/.config/gitlab-mcp/token.json.