standard_init_linux.go:228: exec user process caused: exec format error
I'm actually using the upstream Dockerfile.server file to conduct the build. I've identified the issue. It's due to the way tini is being downloaded/installed. It's explicitly downloading the amd64 version (this is not compatible with armv7 or arm64). I created an issue and verified a fix. The new images are pushed up to Docker Hub if anyone wants to test (works for me).
Issue: https://github.com/laurent22/joplin/issues/6096
PR: https://github.com/laurent22/joplin/pull/6097