We use shell executor.
This reverts commit 1a52e0df.

Image is busybox and do not contains bash or docker.

- Do not try to build missing tarballs
- Tag latest either stable or rc

From https://www.dokuwiki.org/install:upgrade

https://github.com/docker-library/php/issues/931

When cache is enabled, image is not rebuild when executing on the same runner.