I have done nothing but backend for a decade. Docker's been around for the past 6 years and still going strong.
Database? Postgres, always. You have to bend it so far until it breaks it's not even funny. If you need something else, you'll now.
Backend lang? Python. Framework? Django. Need data munging? NumPy/SciPy/Pandas/PySpark. Need a really high speed component? C will do, Rust if you feel fancy.
OS? Ubuntu, or Debian. They'll do well. Again, you need to have really, really good reasons to use anything else.
Of course there's alternatives! But there is absolutely no reason to move away from this stack. For some reason frontend developers seem to be either on a constant treadmill of moving technologies, or being stuck with subpar old tech with clear deficiencies.