fix: crash when creating images having several tags

[dunglas]

Closes #2229.

(*imagetools.Resolver).Copy() can is called in concurrent goroutines.

Because it writes a non-protected map, it sometimes crashes.

[jedevc]

Hey @dunglas thanks for the fix 🎉 🎉 I think I might have beaten you to it sorry! #2230

I'm going to close this one in preference of that one - protecting the map is probably alright, but I think it's better to just avoid modifying one of our inputs in this context at all.

Thanks thanks anyways!