Check contribution documentation, esp. Docker commands

Check the following page: https://doc.zih.tu-dresden.de/contrib/contribute_container/

• Docker command is not working:

taras@rtest:~/hpc-compendium$ export DC='docker exec -it hpc-compendium bash -c '                
taras@rtest:~/hpc-compendium$ echo $DC                                                      
docker exec -it hpc-compendium bash -c 
taras@rtest:~/hpc-compendium$ $DC 'markdown-link-check docs/software/big_data_frameworks.md'
zsh: command not found: docker exec -it hpc-compendium bash -c 

With bash:

taras@rtest:~/hpc-compendium$ sudo docker images
REPOSITORY              TAG            IMAGE ID       CREATED          SIZE
hpc-compendium          latest         3c35392cae30   17 minutes ago   1.56GB
[...]
taras@rtest:~/hpc-compendium$ sudo $DC 'markdown-link-check docs/software/big_data_frameworks.md'
Error: No such container: hpc-compendium

• Other checks are missing, e.g. check wording, check for floating pages (mkdocs serve -s)