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.

Dockerfile.claude

@@ -1,34 +0,0 @@
-FROM node:20-slim
-
-# Install minimal runtime dependencies
-RUN apt-get update && apt-get install -y --no-install-recommends \
-        git \
-        curl \
-        ca-certificates \
-        bash \
-        ttyd \
-    && rm -rf /var/lib/apt/lists/*
-
-# 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 groupadd -g 1000 claude \
-    && useradd -u 1000 -g claude -m -s /bin/bash 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"]