refactor: rebase CI runner on Ubuntu 24.04, rename to ci-base

Replace Alpine-based legion-runner with a minimal Ubuntu 24.04 image.
The act_runner binary is copied from the official image via multi-stage
build. Job-specific tooling (Ruby, Node, Python, etc.) belongs in
per-job container images specified via container: in workflows, not
in the base runner image.

Rename: legion-runner → ci-base

servers/legion/k8s/gitea-runner/Dockerfile

@@ -1,24 +1,11 @@
-FROM gitea/act_runner:latest
+FROM gitea/act_runner:latest AS runner-bin
 
-USER root
+FROM ubuntu:24.04
 
-RUN apk add --no-cache \
-    bash \
-    git \
+COPY --from=runner-bin /bin/act_runner /bin/act_runner
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    ca-certificates \
     curl \
-    nodejs \
-    npm \
-    python3 \
-    py3-pip \
-    python3-dev \
-    gcc \
-    musl-dev \
-    linux-headers \
-    make \
-    ruby \
-    ruby-bundler \
-    ruby-dev \
-    build-base \
-    zlib-dev && \
-    ln -sf /usr/bin/node /usr/local/bin/node && \
-    ln -sf /usr/bin/npm /usr/local/bin/npm
+    git \
+    && rm -rf /var/lib/apt/lists/*