I set up Home Assistant in a Kubernetes container. It needs write access to the /dev/ttyACM0 device on the host computer (i.e. the computer that runs the container). It works if I make the container “privileged” in Docker speak. Kubernetes doesn't give direct access to the Docker engine, but “privileged“ is possible also with Kubernetes.
The problem is that making a container privileged is a measure of last resort. Kubernetes also allows to set Linux capabilities. Now I wonder whether it is possible to grant write access to /dev/ttyACM0 with just (a set of) Linux capabilities?
For what it's worth,
SYS_ADMIN was not enough.