Seems like this question of penetration through cover just keeps coming up, and everybody has a story about what they shot in their backyard, or vaguely recalls some "tests" they saw on YouTube.
I submit for your review, Internet Sheepdogs, an informal 2005 study by the DEA NY Firearms Unit. Bottom line, cars are not good cover.
http://www.anret.org/Downloads/ballistics.pdf
Highlights for those who don't like to read:
-> "..with the exception of buckshot, almost all of the rounds fired..did penetrate and strike the targets inside of the test vehicle."
-> "..the car door did not reliably stop any pistol ammunition and it did not stop any of the rifle ammunition."
-> "Pistol ammunition penetrated both sides of the trunk and the target."
Don't quote me on this, but from a quick search, roll-off dumpsters have wall thickness (in the areas that aren't reinforced) between 3 and 5 millimeters. I'm not sure of the material, I would guess mild steel in most cases, perhaps some are low-grade aluminum.
Also, some food for thought. You've been attacked, and now you're in a gunfight. The violent hoodlum thug, hopped-up on drugs with his sideways Gat and Sox hat to match, gets behind some sort of cover and you, the responsible, mindful, safety-first, internet-reading, Gunsite/Frontsight/Magpul Dynamics trained, law-abiding NRA-supporting gun owner are out in the open, holding your meticulously cleaned and maintained pistol, considering which round to shove into your reload.
Something ain't right if you got yourself in that deep.