fix: default to the buildkit in the caller's own namespace

The default was `tcp://buildkit.gitea.svc.cluster.local:1234`, which
names aceofbase's namespace explicitly. That was correct while aceofbase
was the only cluster running CI, and resolves to nothing from
planet-express — so every workflow moved to the new dink-backed runner
had to override it, and forgetting produced a DNS error from inside
buildx that named neither this action nor the cluster it pointed at.

A BARE Service name resolves in the job container's own namespace, which
is where its runner's buildkit lives on both clusters: `gitea` on
aceofbase, `gitea-runner` on planet-express. One default, correct on
both, and correct for a cluster nobody has built yet.

Verified before changing rather than assumed: on aceofbase the buildkit
Service is in `gitea`, and gitea-act-runner's Role is namespace-scoped
to `gitea`, so its job pods land in the same namespace as the Service.
Existing consumers therefore see no change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C1tzpqzv37DDbtpPTxHsyA

action.yml

@@ -1,15 +1,25 @@
 name: setup-buildx
 description: |
-  Wrapper around docker/setup-buildx-action that defaults to the in-cluster
-  buildkit service running in the gitea namespace of the aceofbase cluster.
-  Lets workflow authors skip the boilerplate of pointing setup-buildx at a
-  remote driver.
+  Wrapper around docker/setup-buildx-action that defaults to the buildkit
+  service in the caller's own namespace. Lets workflow authors skip the
+  boilerplate of pointing setup-buildx at a remote driver.
 
 inputs:
   endpoint:
-    description: 'BuildKit TCP endpoint. Defaults to the in-cluster service.'
+    # A BARE Service name, deliberately, rather than an FQDN. A job container
+    # resolves it in its own namespace, which is where its runner's buildkit
+    # lives on every cluster here: `gitea` on aceofbase, `gitea-runner` on
+    # planet-express. One default is therefore correct on both, and stays
+    # correct for a cluster nobody has built yet.
+    #
+    # It used to default to `tcp://buildkit.gitea.svc.cluster.local:1234`,
+    # which named aceofbase's namespace explicitly. That resolved to nothing
+    # from planet-express, so every workflow moved there had to override it —
+    # and the failure mode was a DNS error inside buildx rather than anything
+    # naming this action.
+    description: 'BuildKit TCP endpoint. Defaults to the buildkit Service in the calling job''s own namespace.'
     required: false
-    default: 'tcp://buildkit.gitea.svc.cluster.local:1234'
+    default: 'tcp://buildkit:1234'
   version:
     description: 'buildx version to install (passed through).'
     required: false