enable gcr and heroku plugins

cmd/drone-docker-heroku/main.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"os"
+	"os/exec"
+	"path"
+)
+
+func main() {
+	var (
+		registry = "registry.heroku.com"
+		process  = os.Getenv("PLUGIN_PROCESS_TYPE")
+		app      = os.Getenv("PLUGIN_APP")
+		email    = os.Getenv("PLUGIN_EMAIL")
+		key      = os.Getenv("PLUGIN_API_KEY")
+	)
+
+	// if the heroku email is provided as a named secret
+	// then we should use it.
+	if os.Getenv("HEROKU_EMAIL") != "" {
+		email = os.Getenv("HEROKU_EMAIL")
+	}
+
+	// if the heroku api key is provided as a named secret
+	// then we should use it.
+	if os.Getenv("HEROKU_API_KEY") != "" {
+		key = os.Getenv("HEROKU_API_KEY")
+	}
+
+	os.Setenv("PLUGIN_REGISTRY", registry)
+	os.Setenv("PLUGIN_REPO", path.Join(registry, app, process))
+
+	os.Setenv("DOCKER_PASSWORD", key)
+	os.Setenv("DOCKER_USERNAME", email)
+	os.Setenv("DOCKER_EMAIL", email)
+
+	// invoke the base docker plugin binary
+	cmd := exec.Command("drone-docker")
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	err := cmd.Run()
+	if err != nil {
+		os.Exit(1)
+	}
+}