Update Dockerfile

[dweinholz]

Try to fulfill the following points before the Pull Request is merged:

• The PR is reviewed by one of the team members.
• If a linting PR exists, it must be merged before this PR is allowed to be merged.
• If the PR is merged in the master then a release should be be made.
• If the new code is readable, if not it should be well commented

For releases only:

• If the review of this PR is approved and the PR is followed by a release then the .env file
  in the cloud-portal repo should also be updated.
• If you are making a release then please sum up the changes since the last release on the release page using the clog tool with clog -F