message: don't ignore DB cache if local cache is empty
  greatly speeds up builds using --skip + docker-compose, for which the local
  cache will _always_ be empty
diff: @@ -44,13 +44,11 @@ impl DocBuilder {
  let path = PathBuf::from(&self.options.prefix).join("cache");
  let reader = fs::File::open(path).map(|f| BufReader::new(f));
  - if reader.is_err() {
  - return Ok(());
  - }
  -
  - for line in reader.unwrap().lines() {
  + if let Ok(reader) = reader {
  + for line in reader.lines() {
  self.cache.insert(line?);
  }
  + }
  self.load_database_cache()?;
  