How do I override the entrypoint and pass bash commands as a string?

I dont want to modify the dockerfile

I want to override the entrypoint and run arbitrary commands but this is not working for me

docker run -it --entrypoint /bin/bash myimage bash -c "echo aaaa &"

I get /bin/bash: /bin/bash: cannot execute binary file

I want to be able to immediately start a background job before entering the container interactively- again without modifying the dockerfile

Source: Docker Questions