A somewhat uneducated remark.
In the US, Canada, and UK there is no obligation to render help if there is no previous patient-doctor relationship
In Aust and Europe, there is an obligation to help even if a previous relationship did not exist
Lufthansa was successfully sued >2million for following the on board doctor's advice of not diverting, when the passenger ultimately suffered a life threatening heart attack
There are naunces with medico-legal law across different countries. Assuming that you will always be seen as the knight in shining armour can bring you unstuck. Coming forward to volunteer your services whilst under the influence is definitely not on, in any country. This question is asked as an ethics question in every job interview for a doctor, and the medico-legal environment is such that the answer is now black and white.
For those interested, some background info that might clear up some questions raised in this thread
Passenger doctors in civil airliners--obligations, duties and standards of care. - PubMed - NCBI
"Passenger doctor in civil airliners — Obligations, duties and standards of care"