Dockerfile: drop libz-dev

[Bo98]

This causes formulae to build against with system zlib which we don't want.

The original need for it is also no longer valid: https://github.com/Homebrew/brew/commit/5e42f6778357f9509f2335c7fe34c5fd5bf5c43a

[Bo98]

Looks like Portable Ruby has a -lz in its rbconfig which I'll need to fix first.

[p-linnane]

@Bo98 Do we need a new Portable Ruby release?

[Bo98]

Yes.

Might be a good opportunity try the Bootsnap thing I've talked about a couple times, and to update the cross-build CI images since we need to do that and I said I would this month.