feat: add support for ssh property in the build command

[banditopazzo]

Fixes #705: Add support for ssh property in the build command

Schema definition: https://github.com/compose-spec/compose-spec/blob/6f87f9328842307d33dc777ca243cf92acf7aea4/schema/compose-spec.json#L107

[p12tic]

I think it would make sense to produce some output that shows that information was really acquired from the agent and then assert it in the test.

[banditopazzo]

do you mean not having the mock agent at all?

[banditopazzo]

sorry, got it, I found a solution to check the key

[p12tic]

Sorry for not being explicit, but your solution was pretty similar to what I was thinking about.

A more simplier solution would be to do:

RUN echo "No id:" >> /result.log 
RUN --mount=type=ssh ssh-add -L >> /result.log

RUN echo "id1:" >> /result.log 
RUN --mount=type=ssh,id=id1 ssh-add -L >> /result.log

RUN echo "id2:" >> /result.log 
RUN --mount=type=ssh,id=id2 ssh-add -L >> /result.log

Then in docker compose set command to cat /result.log and then check the logs in the test via podman compose logs (there are a few tests that do this to use for reference).

[banditopazzo]

Done

[p12tic]

This PR is great, thank you! I had a couple of suggestions for the test, but overall looks great.

[p12tic]

Could you please combine the commits into one and remove feat: prefix as we don't use them in this repository? Thanks!