mxdrw expects HTTP Basic Auth (BASIC_AUTH_USER + BASIC_AUTH_PASS on the
server side). Replace MEXDRAW_TOKEN with MEXDRAW_USER + MEXDRAW_PASS,
use req.SetBasicAuth on the export PUT.
Updated docker-compose.yml comment and README env table to match.
Roundtrip verified locally against mxdrw.msbls.de.