I develop my apps in the most possible native way I can: deb packages, apt repo, systemd, journald etc. however I would like to also be able to run it in docker/vm. Is there a good systemd-in-docker solution for this to basically not run anything differently and not have to maintain two sets of systems?
Containers with systemd as an init process are considered first-class citizen by the Podman ecosystem (the base images are named accordingly: e.g, ubi10-init vs ubi10)
You might be better served by Incus/LXD which run "Linux containers" (ie: a full distro including systemd, SSH etc) as opposed to OCI containers.
https://github.com/Azure/dalec
Build system packages and containers from those packages for a given target distro.
Behind the scenes it uses buildkit, so it's no extra stuff you need, just docker (or any buildkit daemon).
You could use Nix to build the package and provide a nixos module and a docker image from the same derivation. Now you only have to manage three systems instead of two. /s
Have you looked at systemd-nspawn[0]? Its not docker so it wouldn't be useful for writing Dockerfiles but it is light containers that work beautifully with systemd.
[0] https://wiki.archlinux.org/title/Systemd-nspawn