Support `containers` version `0.8`.

[jonathanknowles]

Version 0.8 of the containers package has been released, but indexed-traversable currently has an upper bound of < 0.8:

indexed-traversable/indexed-traversable/indexed-traversable.cabal

Lines 68 to 71 in 645af58

	build-depends:
	array >=0.3.0.2 && <0.6
	, base >=4.12 && <4.22
	, containers >=0.6.0.1 && <0.8