Various fixes
- Use tmux==3.2a because 3.3 has problems with debian 10
- Use debian 11 base Docker image
- Make _find_existing optional trough environment variable (also makes the code reachable for Sonar)
- Add more documentation to generated files (helps with SonarQube as well)
- Replace md5 sum with sha512 sum, so no more complaints about that, and is almost as fast