ci: add docker.io to custom runner images

k8s/infra/forgejo-runner-images/Dockerfile.golang

@@ -1,12 +1,12 @@
 FROM golang:1.26-bookworm
 
-# Add Node.js precheck script
+# Install Node.js, npm, and Docker CLI
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    nodejs npm \
+    nodejs npm docker.io \
     && rm -rf /var/lib/apt/lists/*
 
 # Verify installations
-RUN node --version && npm --version && go version
+RUN go version && node --version && npm --version && docker --version
 
 LABEL maintainer="homelab" \
-      description="Golang 1.26 + Node.js pre-installed with precheck"
+      description="Golang 1.26 + Node.js + Docker CLI pre-installed"