Harden Docker container security

Run as non-root user (fixes Claude Code refusing bypassPermissions as
root, which caused all agent spawns to exit instantly with no session).
Add read-only root filesystem, drop all capabilities, set
no-new-privileges, bind port to localhost only, and require
GIT_USER_NAME/GIT_USER_EMAIL env vars at startup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/entrypoint.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+set -e
+
+# ── Git identity ─────────────────────────────────────────────────────
+# Agents commit code inside the container. Without a git identity,
+# commits fail or use garbage defaults. Fail loudly at startup so the
+# operator knows immediately.
+if [ -z "$GIT_USER_NAME" ]; then
+    echo "FATAL: GIT_USER_NAME is not set. Export it in your environment or docker-compose.yml." >&2
+    exit 1
+fi
+if [ -z "$GIT_USER_EMAIL" ]; then
+    echo "FATAL: GIT_USER_EMAIL is not set. Export it in your environment or docker-compose.yml." >&2
+    exit 1
+fi
+
+# Use GIT_AUTHOR/COMMITTER env vars instead of git config --global,
+# so the root filesystem can stay read-only (no ~/.gitconfig write).
+export GIT_AUTHOR_NAME="$GIT_USER_NAME"
+export GIT_COMMITTER_NAME="$GIT_USER_NAME"
+export GIT_AUTHOR_EMAIL="$GIT_USER_EMAIL"
+export GIT_COMMITTER_EMAIL="$GIT_USER_EMAIL"
+
+exec "$@"