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>

ioutils/temp_unix.go

@@ -0,0 +1,10 @@
+// +build !windows
+
+package ioutils
+
+import "io/ioutil"
+
+// TempDir on Unix systems is equivalent to ioutil.TempDir.
+func TempDir(dir, prefix string) (string, error) {
+	return ioutil.TempDir(dir, prefix)
+}

ioutils/temp_windows.go

@@ -0,0 +1,18 @@
+// +build windows
+
+package ioutils
+
+import (
+	"io/ioutil"
+
+	"github.com/docker/docker/pkg/longpath"
+)
+
+// TempDir is the equivalent of ioutil.TempDir, except that the result is in Windows longpath format.
+func TempDir(dir, prefix string) (string, error) {
+	tempDir, err := ioutil.TempDir(dir, prefix)
+	if err != nil {
+		return "", err
+	}
+	return longpath.AddPrefix(tempDir), nil
+}