Thank you for your elaborate answer Frank! I'm going to make sure all docker containers that create files that can be accessed using samba create them with umask 000. And I will apply the suggested samba configuration. I'm pretty sure this isn't the correct solution from a security perspective, but that doesn't really matter in this case.
I'm a dedicated Linux desktop and server user for more then 20 years, and a software developer. Working with permissions from these different perspectives (os, docker instances, smb server and client, nfs, web hosting and file servers... and especially those services combined on one server) can still throw me off. In this case the user rights are being managed automatically by the os, which makes it even less transparent.
I'm taking the easy way out in this instance, since it's just my home server, and use the solutions you (and others) suggested.
Thanks a bunch!