Plumbing my garage for compressed air...

Copper is fine. Pitch the pipes at least 1/4" per foot, probably away from the compressor, and put a couple of long traps in, with drain cocks at bottom.

Instead of soft solder, consider using what we called "silfoss" used in refrigeration. It's a braze rod used with oxy/ acetylene, acetylene only, or Mapp torches. Needs no flux and is a HELL of a lot stronger than soft solder, and won't corrode or deteriorate

In the summer, you can plumb the outfeed of the compressor to a big coil of copper in a garbage can full of water. This will realy help condense water out of the airflow, but of course you'll need a big trap there as well.