In a comment you clarify:

I'm probably looking for a way to dynamically set the nr of workers somewhere without needing to specify this twice

In the Uvicorn Settings documentation, it notes that environment variables with names beginning with UVICORN_ directly translate to Uvicorn settings, and also that these must be proper environment variables and not from a --env-file. That means, without changing anything at all in your code or your image build, you can set the worker count when you run the container.

docker run -d -p 8080:8080 \
  -e UVICORN_WORKERS=8 \
  my-image

If the variable is already set in the host environment, you can use the -e option without a value.

$ cat .envrc
export UVICORN_WORKERS=8
$ uv run fastapi dev
...
$ docker run -d -p 8080:8080 -e UVICORN_WORKERS my-image

You often won't want to run a dev server in a container. The Uvicorn documentation also has a Docker setup example that suggests uv run uvicorn ... as the main container command

CMD ["uv", "run", "uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]

That's similar to the last Dockerfile fragment you show in that it uses uv run, but also similar to the first fragment in that it directly runs the application server without having a main function in your own code.

The issue was that there is a conflict in uv when using bind mounts. When removing the volumes from the docker-compose file, it worked. I also had to remove a python-version file from my directory. See https://github.com/astral-sh/uv/issues/10615 for more context.