Permit '=' separator and '[ipv6]' in 'extra_hosts'.

[robmry]

Fixes docker/cli#4648

Align the format of extra_hosts strings with updated --add-hosts options in the docker CLI and buildx - by permitting host=ip in addition to host:ip, and allowing square brackets around the address.

For example:

    extra_hosts:
      - "my-host1:127.0.0.1"
      - "my-host2:::1"
      - "my-host3=::1"
      - "my-host4=[::1]"

Added HostsList unit tests for the various input formats.

[ndeloof]

LGTM
a minor note on extending decodeMapping to support both scenarios

[ndeloof]

to promote use of = , HostsList.AsList() should also use it when rendering canonical compose.yaml with docker compose config

[robmry]

to promote use of = , HostsList.AsList() should also use it when rendering canonical compose.yaml with docker compose config

I'm still figuring out how all this stuff fits together - but the engine API won't understand =, it's up to the CLI/buildx/compose clients to convert back to : (less human-readable, but a stable API that avoids problems with an old engine vs. new client).

So, I think, if the docker compose config is what ends up being sent to the API (?), it'll need to stick with the :?