fix(ci): Docker build uses init container + emptyDir instead of PVC

The build-ci-image template used volumeClaimTemplates (PVC) which
weren't inherited when called via templateRef from ci-pipeline.

Restructured to single pod: init container (alpine/git) clones repo
into emptyDir, main container (kaniko) builds and pushes image from
the same volume. No PVC needed, works correctly via templateRef.

Tested: foxhunt-runtime image rebuilt successfully with kubectl.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

infra/k8s/argo/build-ci-image-template.yaml

@@ -75,12 +75,12 @@ spec:
               mountPath: /workspace
           args:
             - |
-              set -e
-              mkdir -p ~/.ssh
-              cp /etc/git-ssh/ssh-privatekey ~/.ssh/id_ed25519
-              chmod 600 ~/.ssh/id_ed25519
-              printf 'Host *\n  StrictHostKeyChecking no\n  UserKnownHostsFile /dev/null\n' > ~/.ssh/config
-              chmod 600 ~/.ssh/config
+              set -ex
+              mkdir -p /root/.ssh
+              cp /etc/git-ssh/ssh-privatekey /root/.ssh/id_ed25519
+              chmod 600 /root/.ssh/id_ed25519
+              printf 'Host *\n  StrictHostKeyChecking no\n  UserKnownHostsFile /dev/null\n' > /root/.ssh/config
+              chmod 600 /root/.ssh/config
 
               SHA="{{inputs.parameters.commit-sha}}"
               REPO="ssh://git@gitlab-gitlab-shell.foxhunt.svc.cluster.local:2222/root/foxhunt.git"