Add build secret flag

[nashiox]

Add the secret flag for build #189

Ex)

- name: build
  image: plugins/docker
  volumes:
  - name: secrets
    path: /var/run/secrets
  environment:
    DOCKER_BUILDKIT: 1
  settings:
    repo: my/repos
    secret: id=mysecret,src=/var/run/secrets/mysecret

[tboerger]

Does it make sense to provide it as a slice? It's possible to add multiple secrets, right?

[nashiox]

Oops, I missed that it was a slice option.
https://github.com/moby/buildkit/blob/master/cmd/buildctl/build.go#L93-L95

Fixed it. 4940a7f

[Weasy666]

Hi! Is there an ETA when this will land?

[davezuch]

Why was this PR closed?

[tboerger]

I guess because it haven't been merged within a year.

[wszychta]

@tboerger @nashiox It there any chance to reopen this issue? We really need this feature to pass .netrc file into Docker build as a temporary secret.