GitHub Enterprise support (#658)

* Add option to specify custom GitHub instance

* Use correct GHE API endpoint URLs

Co-authored-by: Markus Wolf <[email protected]>

* Extract slug from GitHub Enterprise URLs

Co-authored-by: Markus Wolf <[email protected]>

* Use GITHUB_TOKEN for clone authenticate if provided

This change will allow use authentication for cloning actions
from private repositories or github enterprise instances.

Co-Authored-By: Markus Wolf <[email protected]>

* Add section about using act on GitHub Enterprise to README

Co-authored-by: Markus Wolf <[email protected]>

* Set GitHubInstance in runnerConfig in runner_test

Co-authored-by: Markus Wolf <[email protected]>

Co-authored-by: hackercat <[email protected]>
Co-authored-by: Markus Wolf <[email protected]>

cmd/input.go

@@ -29,6 +29,7 @@ type Input struct {
 	containerArchitecture string
 	noWorkflowRecurse     bool
 	useGitIgnore          bool
+	githubInstance        string
 }
 
 func (i *Input) resolve(path string) string {