On one hand, it would obviously attract lightning, being a tall-ish conducter it would attract it, but cars and the like are said to be (relatively) safe specifically because they direct lightning around you to the ground. I imagine it would be similar to that.
is it grounded properly? it would be trivial to install a proper lightning protection on it with a little digging (sheet metal roof might be too thin to conduct lightning current safely and without damage. not a legal advice. consult your local building code)