[builder] Make build cache ignore mtime

Build cache uses pgk/tarsum to get a digest of content which is
ADD'd or COPY'd during a build. The builder has always used v0 of
the tarsum algorithm which includes mtimes however since the whole
file is hashed anyway, the mtime doesn't really provide any extra
information about whether the file has changed and many version
control tools like Git strip mtime from files when they are cloned.

This patch updates the build subsystem to use v1 of Tarsum which
explicitly ignores mtime when calculating a digest. Now ADD and
COPY will result in a cache hit if only the mtime and not the file
contents have changed.

NOTE: Tarsum is NOT a meant to be a cryptographically secure hash
function. It is a best-effort approach to determining if two sets of
filesystem content are different.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Upstream-commit: 0e10507a1c5985e6fda0ff48e9313ba7a4de761b
Component: engine

components/engine/builder/internals.go

@@ -50,7 +50,7 @@ func (b *Builder) readContext(context io.Reader) error {
 		return err
 	}
 
-	if b.context, err = tarsum.NewTarSum(decompressedStream, true, tarsum.Version0); err != nil {
+	if b.context, err = tarsum.NewTarSum(decompressedStream, true, tarsum.Version1); err != nil {
 		return err
 	}
 
@@ -345,7 +345,7 @@ func calcCopyInfo(b *Builder, cmdName string, cInfos *[]*copyInfo, origPath stri
 		if err != nil {
 			return err
 		}
-		tarSum, err := tarsum.NewTarSum(r, true, tarsum.Version0)
+		tarSum, err := tarsum.NewTarSum(r, true, tarsum.Version1)
 		if err != nil {
 			return err
 		}