Inherit extended agent so we get modern sign hashes

[tlbdk]

Closes #1265

@tonistiigi

[tonistiigi]

@tlbdk We should probably disable calling ExtendedAgent.Extension() by masking it in readOnlyAgent, right?

[tlbdk]

@tlbdk We should probably disable calling ExtendedAgent.Extension() by masking it in readOnlyAgent, right?

Hmm, it's a way to extend the protocol so it's not really clear that if it can be used to modify the state of the "host" ssh-agent as it can be used for anything. I guess this is the point of readOnlyAgent. I would leave it in as we don't know if this will be used in the future.

[tonistiigi]

I think it is safer to block it. Don't want this to become a possible security issue. If there is a legitimate case (similar to #1265 for example) we could make a specific exception for that based on the input parameters.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "ssh2-support" git@github.com:tlbdk/buildkit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354500256
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.