Bake: support compose ssh config

[sumnerwarren]

Adds support for compose ssh config with bake. See spec. Based on #1069. This does not remove support for the x-bake ssh field.

[tonistiigi]

fyi @ndeloof

[sumnerwarren]

Looking at the failed check: sorting for multiple ssh keys does not appear to be stable. Do we want bake to make it stable or should we just change to an ElementsMatch assertion?

[tonistiigi]

@sumnerwarren Is it already coming unsorted from compose? In that case, it should be fixed in there. Meanwhile, we can just add some forced sorting in here to get this merged.

[sumnerwarren]

@tonistiigi Yes, it's coming from compose unsorted, at least on macos. I ran docker compose config repeatedly with this MRE compose file to confirm:

services:
  test:
    build:
      ssh:
        - default
        - key=key.pem

I just pushed a commit that includes sorting.

Note: I'm not a go expert, but I think it stems from using range on a map, which does not have a defined sort order.

[crazy-max]

Thanks! Don't think we need a separate commit for sorting, can you squash commits please? Otherwise LGTM