message: Builder: add some comments for workflow
  These comments make the build workflow more clearer.
diff: @@ -242,11 +242,13 @@ impl Bootstrap {
  if ctx.blob_id.is_empty() {
  ctx.blob_id = format!("{:x}", blob_hash.finalize());
  }
  +
  if blob_size > 0 || (ctx.source_type == SourceType::StargzIndex && !ctx.blob_id.is_empty())
  {
  if ctx.prefetch.policy != PrefetchPolicy::Blob {
  blob_readahead_size = 0;
  }
  + // Add new blob to blob table
  ctx.blob_table
  .add(ctx.blob_id.clone(), 0, blob_readahead_size as u32);
  }
  @@ -295,6 +297,7 @@ impl Bootstrap {
  }
  super_block.set_prefetch_table_entries(prefetch_table_entries);
  + // Set inodes and chunks
  let mut inode_offset =
  (super_block_size + inode_table_size + prefetch_table_size + blob_table_size) as u32;
  @@ -319,20 +322,27 @@ impl Bootstrap {
  super_block.set_has_xattr();
  }
  - // Dump bootstrap
  + // Dump super block
  super_block
  .store(&mut ctx.f_bootstrap)
  .context("failed to store superblock")?;
  +
  + // Dump inode table
  inode_table
  .store(&mut ctx.f_bootstrap)
  .context("failed to store inode table")?;
  +
  + // Dump prefetch table
  if let Some(mut prefetch_table) = ctx.prefetch.get_prefetch_table() {
  prefetch_table.store(&mut ctx.f_bootstrap)?;
  }
  +
  + // Dump blob table
  ctx.blob_table
  .store(&mut ctx.f_bootstrap)
  .context("failed to store blob table")?;
  + // Dump inodes and chunks
  timing_tracer!(
  {
  for node in &mut ctx.nodes {
  