Does it work without that UnitTest line in the Dockerfiles ??

 ([ $TESTS -eq 0 ] || iris session iris -U $NAMESPACE "##class(%ZPM.PackageManager).Shell(\"test $MODULE -v -only\",1,1)") && \

Running UnitTest directly in iris.script might be easier to control.
Checking UnitTests with ZPM manually after the first install might give more precise info of the problem

 

My personal approach would be:

1. separate instance code from Class Methods This covers all properties and especially Calculated Properties Or simplified, all code tied to data storage
2.  Class Methods work on referenced objects but have no dedicated data store (i%...) Simplified, they are code-only components.
3.  Class Methods could be bundled in several Classes.