setup guide: better recommendations for Docker usage

[bertsky]

This issue was lost when migrating from docs here:

https://github.com/OCR-D/docs/issues/11#issue-548017304

So, please explain how to use Docker images interactively (without spinning up a container for each individual processor).

Also, now there's a broken link in the setup guide: https://ocr-d.de/en/setup#translating-native-commands-to-docker-calls

[bertsky]

Also, now there's a broken link in the setup guide: https://ocr-d.de/en/setup#translating-native-commands-to-docker-calls

Still true today BTW.

This issue was lost when migrating from docs here:

https://github.com/OCR-D/docs/issues/11#issue-548017304

So, please explain how to use Docker images interactively (without spinning up a container for each individual processor).

Still think this would be useful, especially for first-timers and shell-non-natives (for which using the shell interactively in Docker makes more sense).

[bertsky]

ping @lena-hinrichsen