Allow overriding image creation time

If the SOURCE_DATE_EPOCH env var is set (to a number of seconds since
the Unix epoch), its value will be used as the creation time of the
image produced, and of config history values for each layer.

This matches the behavior of tools like ko[0] and is the recommendation
of https://reproducible-builds.org/

0: https://github.com/google/ko#why-are-my-images-all-created-in-1970

remote/remote.go

@@ -4,6 +4,8 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"os"
+	"strconv"
 	"strings"
 	"time"
 
@@ -629,12 +631,26 @@ func findLayerWithSha(layers []v1.Layer, diffID string) (v1.Layer, error) {
 	return nil, fmt.Errorf("previous image did not have layer with diff id %q", diffID)
 }
 
+func getCreationTimeFromEnv() v1.Time {
+	epoch := os.Getenv("SOURCE_DATE_EPOCH")
+	if epoch == "" {
+		return v1.Time{Time: imgutil.NormalizedDateTime}
+	}
+
+	seconds, err := strconv.ParseInt(epoch, 10, 64)
+	if err != nil {
+		return v1.Time{Time: imgutil.NormalizedDateTime}
+	}
+	return v1.Time{Time: time.Unix(seconds, 0)}
+}
+
 func (i *Image) Save(additionalNames ...string) error {
 	var err error
 
 	allNames := append([]string{i.repoName}, additionalNames...)
 
-	i.image, err = mutate.CreatedAt(i.image, v1.Time{Time: imgutil.NormalizedDateTime})
+	creationTime := getCreationTimeFromEnv()
+	i.image, err = mutate.CreatedAt(i.image, creationTime)
 	if err != nil {
 		return errors.Wrap(err, "set creation time")
 	}
@@ -652,7 +668,7 @@ func (i *Image) Save(additionalNames ...string) error {
 	cfg.History = make([]v1.History, len(layers))
 	for i := range cfg.History {
 		cfg.History[i] = v1.History{
-			Created: v1.Time{Time: imgutil.NormalizedDateTime},
+			Created: creationTime,
 		}
 	}
 

remote/remote_test.go

@@ -1404,6 +1404,30 @@ func testImage(t *testing.T, when spec.G, it spec.S) {
 					h.AssertEq(t, item.Created.Unix(), imgutil.NormalizedDateTime.Unix())
 				}
 			})
+
+			it("can override creation time", func() {
+				img, err := remote.NewImage(repoName, authn.DefaultKeychain)
+				h.AssertNil(t, err)
+
+				tarPath, err := h.CreateSingleFileLayerTar("/new-layer.txt", "new-layer", "linux")
+				h.AssertNil(t, err)
+				defer os.Remove(tarPath)
+
+				h.AssertNil(t, img.AddLayer(tarPath))
+
+				epoch := time.Now().Unix()
+				os.Setenv("SOURCE_DATE_EPOCH", fmt.Sprintf("%d", epoch))
+				h.AssertNil(t, img.Save())
+
+				configFile := h.FetchManifestImageConfigFile(t, repoName)
+
+				h.AssertEq(t, configFile.Created.Time.Unix(), epoch)
+
+				h.AssertEq(t, len(configFile.History), len(configFile.RootFS.DiffIDs))
+				for _, item := range configFile.History {
+					h.AssertEq(t, item.Created.Unix(), epoch)
+				}
+			})
 		})
 
 		when("additional names are provided", func() {