Net Core app won't run on ARm^4

Hi there,
my net core 5 API project builds and runs perfectly on both my Linux boxes,
one is an Intel NUC running Debian 64 bit
and the other is an ARM64 SBC running Armbian.
Whilst I’m able to get the app running in a Docker container on the NUC
it fails on the ARM box with a 'Could not load file or assembly ‘/MyAPI.dll’.
The system cannot find the file specified.
I’ve shelled ( docker exec ) into the container
and the file is there
I’ve also tried launching the dll from the cli of the container and receive the same error !!!
Any ideas guys ?