build, heredoc: show heredoc summary in build output

Buildah must show summary of heredoc content in build output so its easy
for developers to understand which heredoc got executed, this is similar
to what buildkit does for heredoc content.

See: https://github.com/moby/buildkit/blob/master/frontend/dockerfile/dockerfile2llb/convert.go#L1853

Sample output of buildah

```console
STEP 1/5: FROM docker.io/library/python:latest
STEP 2/5: RUN <<EOF (echo "Hello" >> /hello...)
STEP 3/5: RUN python3 <<EOF (with open("/hello", "w") as f:...)
STEP 4/5: RUN ls -a
```

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

imagebuildah/stage_executor.go

@@ -1217,7 +1217,24 @@ func (s *StageExecutor) Execute(ctx context.Context, base string) (imgID string,
 		}
 		logrus.Debugf("Parsed Step: %+v", *step)
 		if !s.executor.quiet {
-			s.log("%s", step.Original)
+			logMsg := step.Original
+			if len(step.Heredocs) > 0 {
+				summarizeHeredoc := func(doc string) string {
+					doc = strings.TrimSpace(doc)
+					lines := strings.Split(strings.ReplaceAll(doc, "\r\n", "\n"), "\n")
+					summary := lines[0]
+					if len(lines) > 1 {
+						summary += "..."
+					}
+					return summary
+				}
+
+				for _, doc := range node.Heredocs {
+					heredocContent := summarizeHeredoc(doc.Content)
+					logMsg = logMsg + " (" + heredocContent + ")"
+				}
+			}
+			s.log("%s", logMsg)
 		}
 
 		// Check if there's a --from if the step command is COPY.

tests/bud.bats

@@ -344,6 +344,13 @@ _EOF
   expect_output --substring "this is the output of test7 part3"
   expect_output --substring "this is the output of test8 part1"
   expect_output --substring "this is the output of test8 part2"
+
+  # verify that build output contains summary of heredoc content
+  expect_output --substring 'RUN <<EOF \(echo "print first line from heredoc"...)'
+  expect_output --substring 'RUN <<EOF \(echo "Heredoc writing first file" >> /file1...)'
+  expect_output --substring 'RUN python3 <<EOF \(with open\("/file2", "w") as f:...)'
+  expect_output --substring 'ADD <<EOF /index.html \(\(your index page goes here))'
+  expect_output --substring 'COPY <<robots.txt <<humans.txt /test/ \(\(robots content)) \(\(humans content))'
 }
 
 @test "bud build with heredoc content which is a bash file" {