Only sentence 1 is correct, although #2 could conceivably occur. here's how they are different:
-" an alcohol-free office" mean an office where there is no alcohol, which would be the more expected meaning
- " an alcoholic-free office" would mean an office where there are no alcoholics, that is, people who are addicted to drinking alcohol. This meaning, while possible, would be a bit strange because you wouldn't ordinarily think about the number of alcoholics in an office
(because this is usually a private matter that might not be known to anyone other than the individual who is an alcoholic; also because generally there wouldn't more than a very few alcoholics in an office).
So, the usual combining form with "free" is a noun, rather than an adjective Thus, we have " smoke-free", "sugar-free", "bacteria-free", and so on.