Restore SourceDateEpoch in tarball for melange (#1310)

We don't use this (and we don't want it) but melange uses the tarball
package, so let's avoid breaking that for now until we can migrate stuff
off of this tangled web of weird fs.FS implementations.

Signed-off-by: Jon Johnson <jon.johnson@chainguard.dev>

pkg/apk/tarball/tarball.go

@@ -20,17 +20,19 @@ import (
 )
 
 type Context struct {
-	SourceDateEpoch time.Time
-	OverrideUIDGID  bool
-	UID             int
-	GID             int
-	OverrideUname   string
-	OverrideGname   string
-	SkipClose       bool
-	UseChecksums    bool
-	remapUIDs       map[int]int
-	remapGIDs       map[int]int
-	overridePerms   map[string]tar.Header
+	SourceDateEpoch    time.Time
+	useSourceDateEpoch bool
+
+	OverrideUIDGID bool
+	UID            int
+	GID            int
+	OverrideUname  string
+	OverrideGname  string
+	SkipClose      bool
+	UseChecksums   bool
+	remapUIDs      map[int]int
+	remapGIDs      map[int]int
+	overridePerms  map[string]tar.Header
 }
 
 type Option func(*Context) error
@@ -52,6 +54,10 @@ func NewContext(opts ...Option) (*Context, error) {
 func WithSourceDateEpoch(t time.Time) Option {
 	return func(ctx *Context) error {
 		ctx.SourceDateEpoch = t
+
+		// to distinguish between unset and zero value
+		ctx.useSourceDateEpoch = true
+
 		return nil
 	}
 }

pkg/apk/tarball/write.go

@@ -148,8 +148,15 @@ func (c *Context) writeTar(ctx context.Context, tw *tar.Writer, fsys fs.FS, user
 		// work around some weirdness, without this we wind up with just the basename
 		header.Name = path
 
-		// zero out timestamps for reproducibility
-		header.ModTime = info.ModTime()
+		// if SourceDateEpoch is set explicitly, overwrite the timestamps, otherwise propagate modtime
+		// (this option is unused in apko but melange uses this, so we support it for now)
+		if c.useSourceDateEpoch {
+			header.AccessTime = c.SourceDateEpoch
+			header.ModTime = c.SourceDateEpoch
+			header.ChangeTime = c.SourceDateEpoch
+		} else {
+			header.ModTime = info.ModTime()
+		}
 
 		if uid, ok := c.remapUIDs[header.Uid]; ok {
 			header.Uid = uid