Prints generated Dockerfile lines nicely (#178)

* Prints generated Dockerfile lines nicely
* Use testhelper for getting porter output

Signed-off-by: Ryan Moran <rmoran@pivotal.io>

pkg/porter/build.go

@@ -77,7 +77,9 @@ func (p *Porter) buildDockerFile() ([]string, error) {
 	lines = append(lines, p.buildPorterSection()...)
 	lines = append(lines, p.buildCMDSection())
 
-	fmt.Fprintln(p.Out, lines)
+	for _, line := range lines {
+		fmt.Fprintln(p.Out, line)
+	}
 
 	return lines, nil
 }

pkg/porter/build_test.go

@@ -39,6 +39,34 @@ func TestPorter_buildDockerfile(t *testing.T) {
 	assert.Equal(t, wantlines, gotlines)
 }
 
+func TestPorter_buildDockerfile_output(t *testing.T) {
+	p := NewTestPorter(t)
+	p.TestConfig.SetupPorterHome()
+	configTpl, err := p.TestConfig.GetPorterConfigTemplate()
+	require.Nil(t, err)
+	p.TestConfig.TestContext.AddTestFileContents(configTpl, config.Name)
+
+	err = p.LoadManifest()
+	require.NoError(t, err)
+
+	// ignore mixins in the unit tests
+	p.Manifest.Mixins = []string{}
+
+	_, err = p.buildDockerFile()
+	require.NoError(t, err)
+
+	wantlines := `
+Generating Dockerfile =======>
+FROM quay.io/deis/lightweight-docker-go:v0.2.0
+FROM debian:stretch
+COPY --from=0 /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
+COPY cnab/ /cnab/
+COPY porter.yaml /cnab/app/porter.yaml
+CMD ["/cnab/app/run"]
+`
+	assert.Equal(t, wantlines, p.TestConfig.TestContext.GetOutput())
+}
+
 func TestPorter_generateDockerfile(t *testing.T) {
 	p := NewTestPorter(t)
 	p.TestConfig.SetupPorterHome()