It would be awesome if you had the official 7.3 image already available on the hub. Is there a deadline to update this version there?
I don’t work for SonarSource but I’m pretty sure the answer is: never.
SonarQube 7.3 was released 3 years ago and it’s not supported. Currently the supported versions are the 8.9 LTS and the latest 9.2.x release. I don’t think they will release a new docker image with unsupported software.