ci: introduce Dockerfile linting

[tiborsimko]

We have recently introduce Dockerfile linting for one repository, see https://github.com/reanahub/reana-env-root6/pull/10

Let's introduce it to all repositories having Dockerfile for consistency and good practices:

• [x] reana-auth-krb5
• [x] reana-auth-vomsproxy
• [x] reana-demo-fcchh-fullsim
• [x] reana-env-aliphysics
• [x] reana-env-jupyter
• [x] reana-env-root6
• [x] reana-job-controller
• [x] reana-message-broker
• [x] reana-server
• [x] reana-ui
• [x] reana-workflow-controller
• [x] reana-workflow-engine-cwl
• [x] reana-workflow-engine-serial
• [x] reana-workflow-engine-yadage
• [x] reana-workflow-monitor
• [x] www.reana.io

[tiborsimko]

All repositories that have a Dockerfile are now being linted by hadolint. Closing.