Add `/app` folder to docker image.

When using the GitHub Action¹, I ran into the error: find: /app/: No such file or directory So the /app directory seems to be missing form the image.

The error is caused by the entrypoint: https://gitlab.com/pipeline-components/org/base-entrypoint/-/blob/master/entrypoint.sh#L30

  if [ "${GITHUB_ACTIONS:-}" = "true" ] ; then
    # problem matchers need to be run from the host
    find /app/ -maxdepth 1 -type f -name '*-problem-matcher.json' -exec cp {} "$PWD" \;
    find . -maxdepth 1 -type f -name '*-problem-matcher.json' \( -exec echo -n '::add-matcher::' \; -a -exec basename {} \;  \)

    # GITHUB escapes too much, so 'unescape' it ;-(
    # shellcheck disable=SC2068
    exec ${@}
  fi

This MR adds the app dir.