Hm… Looking at your other question, you’re possibly unknowingly running a build system—along the lines of make—at runtime (which you generally shouldn’t).
I’ve seen some Makefiles in the past that wanted to turn SHELL into env PATH=$(PATH) $(SHELL), for example.
If you posted your Dockerfile and more of the context from the logs (from when SHELL was unset), then people might be able to help narrow this down more…