gateway: exec API should support sending signals to processes

[tonistiigi]

#1627 added capability to run and exec into containers via gateway API. This was released in v0.8 .

The capability to send/forward signals as part of the API was not part of the initial commit and only has a TODO comment

buildkit/frontend/gateway/client/client.go

Line 78 in ba673bb

// TODO Signal(ctx context.Context, sig os.Signal)

This makes it harder reliably use this API as signals are a very common part of process workflow. For TTY, some cases like closing the process can be worked around with EOF.

We should try to add the missing API asap, hopefully in v0.10 milestone. This is needed if we want to move on with #1472 and missing it would mean that a full buildkit update cycle is needed to test such features.

@coryb

[tonistiigi]

@coryb Do you think you would have time to take this on?

[coryb]

I have done some work on this already, will hopefully have a PR this week.