Is it recommended to run systemd inside docker container?

I'd advise you to avoid systemd in a container if at all possible.

Systemd mounts filesystems, controls several kernel parameters, has its own internal system for capturing process output, configures system swap space, configures huge pages and POSIX message queues, starts an inter-process message bus, starts per-terminal login prompts, and manages a swath of system services. Many of these are things Docker does for you; others are system-level controls that Docker by default prevents (for good reason).

Usually you want a container to do one thing, which occasionally requires multiple coordinating processes, but you usually don't want it to do any of the things systemd does beyond provide the process manager. Since systemd changes so many host-level parameters you often need to run it as --privileged which breaks the Docker isolation, which is usually a bad idea.

As you say in the question, running one "piece" per container is usually considered best. If you can't do this then a light-weight process manager like supervisord that does the very minimum an init process is required to is a better match, both for the Docker and Unix philosophies.

You should think it more to be a question which init system you like to use.

One may use the old /sbin/init or the systemd-daemon running as PID-1 in a container. Any command like "docker stop" will talk to PID-1 only. If you do only have one java application in a container then it is recommended to run that process directly as PID-1 of the container.

Running systemd is mostly not required - if you have multiple services in a container or if some wrapper script uses 'systemctl' then you may still want to use activate it. But the latter use cases would also be covered by docker-systemctl-replacement.

s6 became a somewhat popular init for containers when you need more than one process. And yes, it's not "one process per container", it's "one thing per container". Running a website e.g. is still one thing but it's usually more than one process.