fix: use environment variables for TS_AUTHKEY and TS_LOGIN_SERVER

- docker-compose.yml: TS_AUTHKEY and TS_LOGIN_SERVER now read from environment
- README.md: updated Tailscale configuration instructions to reference env vars instead of editing the file
- Added .gitignore to prevent committing secrets or Tailscale state

This protects reusable test auth keys and custom coordination servers from being committed to any ts-* repo.

.gitignore

@@ -0,0 +1,9 @@
+# Secrets and environment files
+.env
+.env.*
+*.key
+tskey-*
+authkey*
+
+# Tailscale data (never commit state)
+tailscale/tailscale-data/