Note that not all dumpsters on a construction site are trash. The best example is metal, where a full dumpster brings significant money instead of costing to dispose of. When I was working in a construction company, on a regular basis some dudes came along "offering" to empty the metal dumpster for free.
So it's always better to ask first, otherwise it's technically stealing.
So it's always better to ask first, otherwise it's technically stealing.