Print a warning if Gemfile's dependencies are not satisfied

[ntkme]

I observed is that lots of people install Jekyll 4.x locally with their own choices of dependency, test with that locally and check in the Gemfile. Then on GitHub it builds the site using Jekyll 3.x with a different set of dependency. The end results of this is very YMMV. It works sometimes, but in most cases won't. - E.g. #46

To some degree it is a user problem that they are using "jekyll" gem locally and the action runs "github-pages" gem, but this action is named "jekyll-build-pages" so that I think most of the users will get confused by what it actually does.

This PR prints a warning if we detect a Gemfile and found that some dependencies in the Gemfile cannot be met via "github-pages" gem preinstalled in the container, giving user some hints about what might be causing their build failure.

[parkr]

I like this idea generally. Could the user ever see it though if their build doesn't break?

This has been a common issue with Pages since forever since the Gemfile is always ignored.

[ntkme]

I like this idea generally. Could the user ever see it though if their build doesn't break?

This should create an annotation for the run directly on the UI: https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-a-warning-message

[yoannchaudet]

I like that too! Not everyone look at the warnings but they are visible enough we do get reports when our actions issue deprecation warnings.

Will look at the shellcheck issue tomorrow, that does not sound related to this PR.