trunk/86d46891ab87c83114a040159bcc50b04ccf8cd4: [CI] Use a shared registry build cache for docker-builds (#186297)

«Summary docker-builds builds on the remote BuildKit fleet (OSDC) and passed --no-cache, so every image rebuilt all layers each run — and the ephemeral pods can't share their pod-private caches anyway. This replaces it (r...»