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/webui-entrypoint.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# Entrypoint for the webui service.
+# Wraps Claude Code in ttyd (terminal-over-WebSocket) with basic auth.
+set -euo pipefail
+
+: "${WEBUI_PASSWORD:?WEBUI_PASSWORD must be set in .env}"
+WEBUI_USER="${WEBUI_USER:-claude}"
+WEBUI_PORT="${WEBUI_PORT:-7681}"
+
+exec ttyd \
+    --port "${WEBUI_PORT}" \
+    --writable \
+    --credential "${WEBUI_USER}:${WEBUI_PASSWORD}" \
+    claude