fix(ci): add /tmp to kaniko-act (r2)

kaniko's scratch image has no /tmp; the build script and act's
RUNNER_TEMP both assume it exists.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ci/Dockerfile.kaniko-act

@@ -1,8 +1,9 @@
 # Kaniko executor adapted for Gitea act_runner job containers.
 #
-# act starts job containers with entrypoint /bin/sleep; kaniko's image
-# already symlinks /bin/sh -> /busybox/sh but ships no /bin/sleep.
-# This wrapper adds that one symlink. Nothing else changes.
+# act starts job containers with entrypoint /bin/sleep and sets
+# RUNNER_TEMP=/tmp; kaniko's scratch-based image has neither /bin/sleep
+# (busybox lives under /busybox; /bin/sh is already symlinked) nor /tmp.
+# This wrapper adds exactly those two. Nothing else changes.
 #
 # Bootstrap: the first build of this image is done manually on the host
 # (docker build -f ci/Dockerfile.kaniko-act -t <registry>/mikkeli/kaniko-act:<ver> .)
@@ -10,4 +11,4 @@
 # the build-image workflow itself, using the previous kaniko-act.
 FROM gcr.io/kaniko-project/executor:v1.23.2-debug
 SHELL ["/busybox/sh", "-c"]
-RUN ln -sf /busybox/sleep /bin/sleep
+RUN ln -sf /busybox/sleep /bin/sleep && mkdir -p -m 1777 /tmp