hexagon: check-tcg rebuilding up to date image

[Matheus Tavares Bernardino]

Hi,

We noticed that local `make check-tcg` is rebuilding the docker image
for qemu-hexagon at every run, whereas previously it would say "Image is
up to date" and move on.

This was changed at 0b1a649047 (tests/docker: use direct RUNC call to
build containers, 2023-02-28), where we started to no longer use
docker.py and its image_matches_dockerfile() to skip image builds.

Is this new behavior by design? Or perhaps do we have some local
docker misconfiguration that is not correctly using caches?
 
Thanks,
Matheus