port --secret and --ssh support

[alefray]

This PR resolves #198

The current implementation of img build does not support --secret and --ssh arguments nor accept RUN --mount=... instruction.
These features are now in GA since docker 20.10 and would be a great addition to img

This work is largely inspired from:

• https://github.com/moby/buildkit/blob/master/cmd/buildctl/build.go
• https://github.com/docker/cli/blob/master/cli/command/image/build_buildkit.go

Few comments about this work:

• moby/buildkit must be built with tags dfrunmount dfsecrets dfssh for --secret and --ssh support
• A private key has been added for TestBuildSsh. I'm not sure if this was the best way to make a unit test for --ssh

References:

• https://medium.com/@tonistiigi/build-secrets-and-ssh-forwarding-in-docker-18-09-ae8161d066
• https://medium.com/nttlabs/docker-20-10-59cc4bd59d37

[AkihiroSuda]

These tags should not be needed if you vendor the latest version of BuildKit

[alefray]

After trying a bit, vendoring the latest (8.1.0) version of BuildKit requires much more effort as it impacts refactoring "pull" and "push" commands due to changes on sessions (e.g., moby/buildkit#1551).

Wouldn't it be better if we open a dedicated issue/PR ?

[gabyx]

Would it be possible to support id=git,env=PASSWORD as well?