build: add docker.io to golang runner base image

Pre-install docker.io so golang projects don't need to install on every build.
Avoids duplicating 'apt-get install docker.io' in every CI workflow.

k8s/infra/forgejo-runner/Dockerfile.golang

@@ -1,10 +1,11 @@
 FROM code.forgejo.org/forgejo/runner:6
 
-# Install Node.js (needed for GitHub Actions checkout@v4, etc.)
+# Install Node.js + Docker client (needed for GitHub Actions + docker build)
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
       nodejs \
-      npm && \
+      npm \
+      docker.io && \
     rm -rf /var/lib/apt/lists/*
 
 # Verify installations