code: async function getTagCommit(githubRepo, tag) {
    const cachedResult = await renovateCache.get(
      cacheNamespace,
      getCacheKey(githubRepo, `tag-${tag}`)
    );
    // istanbul ignore if
    if (cachedResult) {
      return cachedResult;
    }
    let digest;
    try {
      const url = `https://api.github.com/repos/${githubRepo}/git/refs/tags/${tag}`;
      digest = (await ghGot(url)).body.object.sha;
    } catch (err) {
      logger.info(
        { githubRepo, err },
        'Error getting tag commit from GitHub repo'
      );
    }
    if (!digest) {
      return null;
    }
    const cacheMinutes = 120;
    await renovateCache.set(
      cacheNamespace,
      getCacheKey(githubRepo, `tag-${tag}`),
      digest,
      cacheMinutes
    );
    return digest;
  }
docs: /*
  github.getDigest
  
  The `newValue` supplied here should be a valid tag for the docker image.
  
  This function will simply return the latest commit hash for the configured repository.