build: add SSH agent socket forwarder (docker build --ssh $SSHMOUNTID=$SSH_AUTH_SOCK)

Unlike `docker build --secret`, `docker build --ssh` allows the build container to
use SSH keys with passphrases.

  $ eval $(ssh-agent)
  $ ssh-add ~/.ssh/id_rsa
  (Input your passphrase here)
  $ docker build --ssh default=$SSH_AUTH_SOCK ...

This feature requires the daemon with `CapExecMountSSH` build capability (moby/moby#37973) .

Currently, the official Dockerfile frontend does not provide the syntax for using the SSH forwarder.

However, the experimental `RUN --mount=type=ssh` syntax can be enabled by using
the Dockerfile frontend image built with the `BUILDTAGS="dfrunmount dfssh"`, via the `# syntax =` "shebang".

The Dockerfile for the Dockerfile frontend is available at  github.com/moby/buildkit/frontend/dockerfile/cmd/dockerfile-frontend)
The pre-built image is also available as `tonistiigi/dockerfile:ssh20181002` .

An example Dockerfile with `RUN --mount=type=ssh`:

  # syntax = tonistiigi/dockerfile:ssh20181002
  FROM alpine
  RUN apk add --no-cache openssh-client
  RUN mkdir -p -m 0700 ~/.ssh && ssh-keyscan gitlab.com >> ~/.ssh/known_hosts
  RUN --mount=type=ssh ssh git@gitlab.com | tee /hello
  # "Welcome to GitLab, @GITLAB_USERNAME_ASSOCIATED_WITH_SSHKEY" should be printed here

More info available at moby/buildkit#608, moby/buildkit#655

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

docs/reference/commandline/build.md

@@ -59,6 +59,7 @@ Options:
                                 Unit is optional and can be `b` (bytes), `k` (kilobytes), `m` (megabytes),
                                 or `g` (gigabytes). If you omit the unit, the system uses bytes.
       --squash                  Squash newly built layers into a single new layer (**Experimental Only**)
+      --ssh                     SSH agent socket or keys to expose to the build (only if BuildKit enabled) (format: default|<id>[=<socket>|<key>[,<key>]])
   -t, --tag value               Name and optionally a tag in the 'name:tag' format (default [])
       --target string           Set the target build stage to build.
       --ulimit value            Ulimit options (default [])