Add github-token input to readme

[polarctos]

Version v4.5.0 introduced usage of the GitHub token for downloads.

Unfortunately this change was not backwards compatible on GitHub Enterprise instances (<company>.ghe.com) as the downloads require a token for github.com, and not the token from the dedicated GHE server/cloud which is present in github.token. Maybe a new major v5 would have been nice.

This PR just adds the github-token input to the readme to make it easier for GHE users to discover this input and to pass in a github.com token e.g. from a secret.

[crazy-max]

It was not put in the readme on purpose as the github token is not required:

setup-docker-action/action.yml

Line 40 in efe9e38

required: false

Does it fail on your side? If so can you show some logs? Thanks!

[polarctos]

When running on a action triggered from GitHub Enterprise Cloud, the error in the action log is: (with v4.5.0)

Download docker
  Downloading Docker latest from stable at download.docker.com
Error: Failed to get Docker releases from https://raw.githubusercontent.com/docker/actions-toolkit/main/.github/docker-releases.json with status code 404: 404: Not Found

In v4.4.0 the action works fine too.

When setting the input to a github.com token the download works fine too.

The main learning here is that ${{ github.token }} is not always a token for github.com but can also be for <>.ghe.com.

https://docs.github.com/en/enterprise-cloud@latest/admin/data-residency/network-details-for-ghecom

[polarctos]

I think it is completely fine to rely on the default ${{ github.token }}, as this will do it for most users. And as the input is also available, the GHE users can also make it work, as it is the same requirement for lots of other setup-* actions. Having all the inputs in the readme too, makes it more convenient when setting the action up.