bake: sort extra-hosts before generating build options

[crazy-max]

fixes #3788

This change makes bake emit extra-hosts in a stable order before passing them into build options.

[thaJeztah]

Is the problem that the bake file uses a map (or similar)? Curious if we have a way to preserve the order in this they are defined. It may be a corner case, but (eg) in this case, order may matter because I think the first match is used;

127.0.0.1   example.com
192.168.1.10 example.com

[blowfishpro]

@thaJeztah FWIW yes, extra-hosts is a mapping of hostname -> IP address, so it would be impossible to add duplicates, and any order information is lost when it's unmarshaled to a golang map. I took a quick look at the HCL package and it looks like it does at least at some level parse maps as an ordered sequence of key-value pairs, but I have no idea how much it would be to leverage that.