Docker Error standard_init_linux.go:185: exec user process caused "exec format error with Qnap TS131P
The problem was indeed that Docker image files are architecture specific (at least the default ones). So a Docker file built on Intel will only work on Intel, and a Docker file build for Arm32 will only work for Arm32.
It seems there are ways to build an Arm build on an Intel device but that would still leave you with distributing two separate images. And if you have a physical Arm device it is much easier to build an Arm image directly on an Arm device.
You also need to make sure your base image supports your architecture, but the official one have now been built as multi-arch images so this is not usually a problem.