revert Dockerfile FROM to gitea agent-base; keep push-target switch

The buildkit builder in the k8s runner cannot pull FROM the artifactapi
registry (internal CA is not trusted). Keep the base-image FROM on gitea;
only the push target moves to artifactapi.

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

.woodpecker/build.yaml

@@ -7,4 +7,3 @@ steps:
     settings:
       repo: artifactapi.k8s.syd1.au.unkin.net/docker-internal/agent-dev
       dry_run: true
-      insecure_skip_verify: true

Dockerfile

@@ -1,4 +1,4 @@
-FROM artifactapi.k8s.syd1.au.unkin.net/docker-internal/agent-base:latest
+FROM git.unkin.net/unkin/agent-base:latest
 
 USER root