refactor(docker): migrate both images to Alpine

Replace node:20-slim/ubuntu:22.04 with node:20-alpine/alpine:3.21.
Switch package management from apt to apk (--no-cache, no cleanup layer).
Use Alpine addgroup/adduser in claude/Dockerfile. Update proxy to use
squid user (Alpine convention) and /var/cache/squid cache path.
Fix proxy/Dockerfile COPY path now that context is proxy/. Move
webui-entrypoint.sh into claude/ to match its build context. Fix
docker-compose.yml webui context to claude/, update proxy tmpfs path.

claude/Dockerfile

@@ -0,0 +1,33 @@
+FROM node:20-alpine
+
+# Install runtime dependencies
+RUN apk add --no-cache \
+        git \
+        curl \
+        ca-certificates \
+        bash \
+        ttyd
+
+# Entrypoint used by the webui service (ttyd wrapping claude)
+COPY --chmod=755 webui-entrypoint.sh /usr/local/bin/webui-entrypoint.sh
+
+# Create non-root user
+RUN addgroup -g 1000 claude \
+    && adduser -u 1000 -G claude -s /bin/bash -D claude
+
+# Install Claude Code globally (runs as root for npm -g, then drops)
+RUN npm install -g @anthropic-ai/claude-code
+
+# Workspace directory owned by claude user
+RUN mkdir -p /workspace && chown claude:claude /workspace
+
+USER claude
+WORKDIR /workspace
+
+# Proxy traffic through sidecar — override at runtime if needed
+ENV HTTP_PROXY=http://proxy:3128
+ENV HTTPS_PROXY=http://proxy:3128
+ENV ALL_PROXY=http://proxy:3128
+ENV NO_PROXY=localhost,127.0.0.1
+
+ENTRYPOINT ["claude"]