refactor(docker): read Dockerfile from stdin (#16)

.gitignore

@@ -1,3 +1,2 @@
 coverage.txt
-Dockerfile
 vendor

conform/enforce.go

@@ -3,7 +3,6 @@ package conform
 import (
 	"bytes"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"os/exec"
 	"strconv"
@@ -55,7 +54,7 @@ func NewEnforcer(rule string) (enforcer *Enforcer, err error) {
 }
 
 // ExecuteBuild executes a docker build.
-func (e *Enforcer) ExecuteBuild() error {
+func (e *Enforcer) ExecuteBuild(dockerfile string) error {
 	image := e.FormatImageNameSHA()
 	if e.GitInfo.IsDirty {
 		image = e.FormatImageNameDirty()
@@ -66,8 +65,9 @@ func (e *Enforcer) ExecuteBuild() error {
 		return err
 	}
 
-	args := append([]string{"build", "--tag", image, "."})
+	args := append([]string{"build", "--tag", image, "-f", "-", "."})
 	command := exec.Command("docker", args...)
+	command.Stdin = strings.NewReader(dockerfile)
 	command.Stdout = os.Stdout
 	command.Stderr = os.Stderr
 	err = command.Start()
@@ -80,28 +80,21 @@ func (e *Enforcer) ExecuteBuild() error {
 }
 
 // RenderDockerfile writes the final Dockerfile to disk.
-func (e *Enforcer) RenderDockerfile(target *config.Rule) error {
-	dockerfile := `# THIS FILE IS AUTOGENERATED BY CONFORM. DO NOT EDIT!
-# Note: This file should be ignored by git.
-  `
+func (e *Enforcer) RenderDockerfile(target *config.Rule) (dockerfile string, err error) {
 	for _, p := range target.Templates {
-		r, err := e.RenderTemplate(p)
-		if err != nil {
-			return err
+		r, _err := e.RenderTemplate(p)
+		if _err != nil {
+			err = _err
+			return
 		}
 		dockerfile = dockerfile + "\n" + *r
 	}
 
 	if e.config.Debug {
 		fmt.Println(dockerfile)
-	} else {
-		err := ioutil.WriteFile("Dockerfile", []byte(dockerfile), 0644)
-		if err != nil {
-			return err
-		}
 	}
 
-	return nil
+	return
 }
 
 // RenderTemplate executes the template and returns it.
@@ -163,11 +156,11 @@ func (e *Enforcer) ExecuteRule() error {
 				return err
 			}
 		}
-		err := e.RenderDockerfile(t)
+		dockerfile, err := e.RenderDockerfile(t)
 		if err != nil {
 			return err
 		}
-		err = e.ExecuteBuild()
+		err = e.ExecuteBuild(dockerfile)
 		if err != nil {
 			return err
 		}