Exec format error, when running 19.07.03 rpi3 image in docker (on rpi3)

Hi everyone, I 've been playing with OpenWRT as a travel router on one of my Raspberry Pi 3s and ran into a weird issue: Since I wanted the RasPi to provide a few other services (audio streaming and a pihole) in parallel to its services as a router, I tried to go for OpenWRT in a docker container. (Yes, probably that doesn't make too much sense. Sorry. I had to try.)

First I googled, found some instructions (eg this one by oofnikj) and tried to follow them. True to the instructions, I downloaded the openwrt-19.07.3-brcm2708-bcm2710-rpi-3-ext4-factory.img.gz image from the openwrt site - but then I kept getting errors when the "docker build" command tried to execute commands in the container:

standard_init_linux.go:211: exec user process caused "exec format error"

So the rpi3 container was not meant for the rpi3 architecture? Weird. I considered it a docker config issue (--> probably my fault) and flashed the same image to a sd card where it worked right fine with the same RasPi board.

After some days of playing around, I decided to try to build the docker image from the running sd card with a fresh docker install (this time "by the book"). The build ran fine - until I started the container, when the console told me...

standard_init_linux.go:211: exec user process caused "exec format error"

I sighed, and assumed the same docker config error. Out of pure curiosity, however, I downloaded the rpi2 image (openwrt-19.07.3-brcm2708-bcm2709-rpi-2-ext4-factory.img.gz) and tried to dockerize it again - and this time it worked out of the box!

Yes, I checked, and my board is a Raspberry Pi 3 B from 2015 - at least that's what's printed on the board. (Also it's got wifi and stuff onboard - so I'm pretty sure.)

Could it be that the 19.07.03 rpi3 image has a special property that tells my docker service to consider it the wrong architecture?

I'm baffled. (But at least I can use OpenWRT via the bare-metal install.)

Any ideas?