Resolve "docker post TOC seems broken"

This fixes the layout of the summary box on the Docker post series. See

• https://hifis-review-app.hzdr.de/review-apps/hifis-hifis-net/144-docker-post-toc-seems-broken/tutorial/2020/09/29/getting-started-with-docker-3
• https://hifis-review-app.hzdr.de/review-apps/hifis-hifis-net/144-docker-post-toc-seems-broken/tutorial/2020/09/25/getting-started-with-docker-2 and
• https://hifis-review-app.hzdr.de/review-apps/hifis-hifis-net/144-docker-post-toc-seems-broken/tutorial/2020/09/23/getting-started-with-docker-1

@jandt-desy Please have a look!

Closes #144 (closed)