fix: use TS_AUTHKEY and TS_LOGIN_SERVER environment variables

- docker-compose.yml: read authkey and login server from env
- README.md: updated instructions to use env vars instead of editing the file
- Added .gitignore for secrets and Tailscale state

.gitignore

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