Opinionated, yes. Monolithic, no. Huge mess of everything that deeply integrates in any system — of course not, your containers don't need to know anything about Docker and host system, you are absolutely free in choices. It's even possible to run (gasp!) multiple services with supervision inside Docker.
That's the first time I ever heard anybody argue that Docker isn't monolothic. It does everything inside it's single daemon executable. Compare against Rocket, which doesn't use a daemon, and uses separate executables for different tasks and stages.
Docker is monolithic, probably more so than it should be, but designed quite well, allowing for things like triton to exist. Systemd is monolithic, but does far more than docker, and really more than it should.
