Security headers not set in vantage6-UI

vantage6-UI is the official user interface for the vantage6 server. In affected versions a number of security headers are not set. This issue has been addressed in commit `68dfa6614` which is expected to be included in future releases. Users are advised to upgrade when a new release is made. While an upgrade path is not available users may modify the docker image build to insert the headers into nginx.