Similar signs have been found in Arizona, as witnessed by my friend Manny, and an ASU student who shot the following video:
I recall that Manny was hoping to research the group behind the signs, so perhaps he'll chime in and offer some insight. From what I've been able to gather, this Citizens for Public Awareness organization doesn't exactly have much presence on the internet. That said, what little info I've been able to dredge up suggests that its founder and president, Nancy Schaefer, has been known to make nativist statements in the past. So, assuming this is the same group, the signs are definitely in character.
I'll likely be making a few phone calls Monday to see who has jurisdiction over dumpsters, and see what it'll take to get the signs removed.
All that aside, there is something fittingly symbolic of racist garbage like the above signs glued to dumpsters.
For those requiring some context, here's a little something on nativism.
Cross-posted from The Mahatma X Files.