Abstract builder and implement server-side dockerfile builder

This patch creates interfaces in builder/ for building Docker images.
It is a first step in a series of patches to remove the daemon
dependency on builder and later allow a client-side Dockerfile builder
as well as potential builder plugins.

It is needed because we cannot remove the /build API endpoint, so we
need to keep the server-side Dockerfile builder, but we also want to
reuse the same Dockerfile parser and evaluator for both server-side and
client-side.

builder/dockerfile/ and api/server/builder.go contain implementations
of those interfaces as a refactoring of the current code.

Signed-off-by: Tibor Vass <tibor@docker.com>
Upstream-commit: e0ef11a4c2c6789e08b61df082b5b9aa70898e7a
Component: engine

components/engine/api/client/build.go

@@ -12,7 +12,6 @@ import (
 	"net/url"
 	"os"
 	"os/exec"
-	"path"
 	"path/filepath"
 	"regexp"
 	"runtime"
@@ -131,13 +130,19 @@ func (cli *DockerCli) CmdBuild(args ...string) error {
 		return fmt.Errorf("cannot canonicalize dockerfile path %s: %v", relDockerfile, err)
 	}
 
-	var includes = []string{"."}
-
-	excludes, err := utils.ReadDockerIgnore(path.Join(contextDir, ".dockerignore"))
-	if err != nil {
+	f, err := os.Open(filepath.Join(contextDir, ".dockerignore"))
+	if err != nil && !os.IsNotExist(err) {
 		return err
 	}
 
+	var excludes []string
+	if err == nil {
+		excludes, err = utils.ReadDockerIgnore(f)
+		if err != nil {
+			return err
+		}
+	}
+
 	if err := utils.ValidateContextDirectory(contextDir, excludes); err != nil {
 		return fmt.Errorf("Error checking context: '%s'.", err)
 	}
@@ -149,6 +154,7 @@ func (cli *DockerCli) CmdBuild(args ...string) error {
 	// removed.  The deamon will remove them for us, if needed, after it
 	// parses the Dockerfile. Ignore errors here, as they will have been
 	// caught by ValidateContextDirectory above.
+	var includes = []string{"."}
 	keepThem1, _ := fileutils.Matches(".dockerignore", excludes)
 	keepThem2, _ := fileutils.Matches(relDockerfile, excludes)
 	if keepThem1 || keepThem2 {