Merge branch 'docker' into 'master'

Replace davidhrbac/docker-mdcheck with it4innovations/docker-mdcheck

See merge request !4

.gitlab-ci.yml

 docs:
   stage: test
-  image: davidhrbac/docker-mdcheck:latest
-  allow_failure: true
+  image: it4innovations/docker-mdcheck:latest
   script:
   - mdl -r ~MD013 *.md
 
 shellcheck:
   stage: test
-  image: davidhrbac/docker-shellcheck:latest
+  image: it4innovations/docker-shellcheck:latest
   allow_failure: true
   script:
-  - which shellcheck || apt-get update && apt-get install -y shellcheck
   - shellcheck eb scripts/*.sh scripts/lib/*