examples: update guppy example with latest fn functionalities (#432)

* examples: update guppy example

* examples: fix typo

fn/langs/ruby.go

@@ -1,5 +1,13 @@
 package langs
 
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
 type RubyLangHelper struct {
 }
 
@@ -8,14 +16,42 @@ func (lh *RubyLangHelper) Entrypoint() string {
 }
 
 func (lh *RubyLangHelper) HasPreBuild() bool {
-	return false
+	return true
 }
 
-// PreBuild for Go builds the binary so the final image can be as small as possible
 func (lh *RubyLangHelper) PreBuild() error {
+	wd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+
+	if !exists(filepath.Join(wd, "Gemfile")) {
+		return nil
+	}
+
+	pbcmd := fmt.Sprintf("docker run --rm -v %s:/worker -w /worker iron/ruby:dev bundle install --standalone --clean", wd)
+	fmt.Println("Running prebuild command:", pbcmd)
+	parts := strings.Fields(pbcmd)
+	head := parts[0]
+	parts = parts[1:len(parts)]
+	cmd := exec.Command(head, parts...)
+	cmd.Stderr = os.Stderr
+	cmd.Stdout = os.Stdout
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("error running docker build: %v", err)
+	}
 	return nil
 }
 
 func (lh *RubyLangHelper) AfterBuild() error {
 	return nil
 }
+
+func exists(name string) bool {
+	if _, err := os.Stat(name); err != nil {
+		if os.IsNotExist(err) {
+			return false
+		}
+	}
+	return true
+}