stage_executor,layers: burst cache if heredoc content is changed

When using buildah with `--layers` then buildah must correctly burst
layer cache if `heredoc` content is changed. Following is achieved via
properly adding `heredoc` content to the history of the built image.

Closes: #5225

Signed-off-by: flouthoc <flouthoc.git@gmail.com>

Author: flouthoc

imagebuildah/stage_executor.go

@@ -1725,7 +1725,14 @@ func (s *StageExecutor) getCreatedBy(node *parser.Node, addedContentSummary stri
 		if buildArgs != "" {
 			return "|" + strconv.Itoa(len(strings.Split(buildArgs, " "))) + " " + buildArgs + " /bin/sh -c " + node.Original[4:]
 		}
-		return "/bin/sh -c " + node.Original[4:]
+		result := "/bin/sh -c " + node.Original[4:]
+		if len(node.Heredocs) > 0 {
+			for _, doc := range node.Heredocs {
+				heredocContent := strings.TrimSpace(doc.Content)
+				result = result + "\n" + heredocContent
+			}
+		}
+		return result
 	case "ADD", "COPY":
 		destination := node
 		for destination.Next != nil {

tests/bud.bats

@@ -293,6 +293,40 @@ _EOF
   run_buildah 1 run myctr ls -l subdir/
 }
 
+@test "bud --layers should not hit cache if heredoc is changed" {
+  local contextdir=${TEST_SCRATCH_DIR}/bud/platform
+  mkdir -p $contextdir
+
+  cat > $contextdir/Dockerfile << _EOF
+FROM alpine
+RUN <<EOF
+echo "Cache burst" >> /hello
+EOF
+RUN cat hello
+_EOF
+
+  # on first run since there is no cache so `Cache burst` must be printed
+  run_buildah build $WITH_POLICY_JSON --layers -t source -f $contextdir/Dockerfile
+  expect_output --substring "Cache burst"
+
+  # on second run since there is cache so `Cache burst` should not be printed
+  run_buildah build $WITH_POLICY_JSON --layers -t source -f $contextdir/Dockerfile
+  # output should not contain cache burst
+  assert "$output" !~ "Cache burst"
+
+  cat > $contextdir/Dockerfile << _EOF
+FROM alpine
+RUN <<EOF
+echo "Cache burst add diff" >> /hello
+EOF
+RUN cat hello
+_EOF
+
+  # on third run since we have changed heredoc so `Cache burst` must be printed.
+  run_buildah build $WITH_POLICY_JSON --layers -t source -f $contextdir/Dockerfile
+  expect_output --substring "Cache burst"
+}
+
 @test "bud build with heredoc content" {
   run_buildah build -t heredoc $WITH_POLICY_JSON -f $BUDFILES/heredoc/Containerfile .
   expect_output --substring "print first line from heredoc"