chore(k8s): take the image reference from build-tools (#196)

## Summary

`k8s/deploy.yaml` takes the image from build-tools: `image: ${IMAGE}` instead of the hard-coded `oci.unbound.se/dancefinder/dancefetcher:${COMMIT}`.

`deploy` substitutes `${IMAGE}` with `<registry>/<owner>/<name>:<tag>`, where the registry comes from the build-tools config and the tag is the value `${COMMIT}` gets today. The rendered reference is unchanged. Moving off `oci.unbound.se` later becomes one change in infra.

Validated on shiny/time-service#864: merged and deployed to AWS staging and Frostmoln staging with the same image. Prerequisites, all merged:
- unboundsoftware/infra#1808: registry in `BUILDTOOLS_FROSTMOLN_STAGING`
- shiny/acctest#952: acctest pins build-tools to the Gitea registry

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://gitea.unbound.se/dancefinder/dancefetcher/pulls/196

k8s/deploy.yaml

@@ -13,7 +13,7 @@ spec:
         spec:
           containers:
             - name: dancefetcher
-              image: oci.unbound.se/dancefinder/dancefetcher:${COMMIT}
+              image: ${IMAGE}
               imagePullPolicy: Always
               resources:
                 limits: