Normally, professions are used without article when using the verb être:
Il/Elle est médecin.
However, I've read that when the profession is modified, the pronoun il/elle should be replaced by ce:
C'est un bon médecin. (Not: Il/Elle est un bon médecin.)
So how can I say the following in French?
You are a good doctor.
According to the rules above, saying "Vous êtes un bon médecin" would be wrong. But saying "C'est un bon médecin" doesn't sound right in this case either.
Similarly, how could one say "I am a good doctor"?