docker: kube-conformance-{release tag} wrongfully created for validation releases

kube-conformance is atipical, since instead of '-latest', it is tagged with the actual kube-conformance upstream version (currently v1.16) - so we build and tag [2] every night.

However, for tagged validation releases, the upstream tag is wrongly overwritten by the validation tag (currently 3.0.0), resulting in building and pushing [1].

[1] https://hub.docker.com/layers/akraino/validation/kube-conformance-3.0.0/images/sha256-82f73e593fe2a7d552049c61cf371a911334c83acf0228b62ec40b46e9889337?context=explore

[2] https://hub.docker.com/layers/akraino/validation/kube-conformance-v1.16/images/sha256-f0976d0c2ee3c4062dc0bf7f880620d8ac2a4e04a6e319b45ffec74d0e1004b8?context=explore