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>
(cherry picked from commit db7399a016)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/image/build.go

@@ -73,6 +73,7 @@ type buildOptions struct {
 	platform       string
 	untrusted      bool
 	secrets        []string
+	ssh            []string
 }
 
 // dockerfileFromStdin returns true when the user specified that the Dockerfile
@@ -158,6 +159,9 @@ func NewBuildCommand(dockerCli command.Cli) *cobra.Command {
 
 	flags.StringArrayVar(&options.secrets, "secret", []string{}, "Secret file to expose to the build (only if BuildKit enabled): id=mysecret,src=/local/secret")
 	flags.SetAnnotation("secret", "version", []string{"1.39"})
+
+	flags.StringArrayVar(&options.ssh, "ssh", []string{}, "SSH agent socket or keys to expose to the build (only if BuildKit enabled) (format: default|<id>[=<socket>|<key>[,<key>]])")
+	flags.SetAnnotation("ssh", "version", []string{"1.39"})
 	return cmd
 }
 

cli/command/image/build_buildkit.go

@@ -27,6 +27,7 @@ import (
 	"github.com/moby/buildkit/session/auth/authprovider"
 	"github.com/moby/buildkit/session/filesync"
 	"github.com/moby/buildkit/session/secrets/secretsprovider"
+	"github.com/moby/buildkit/session/sshforward/sshprovider"
 	"github.com/moby/buildkit/util/appcontext"
 	"github.com/moby/buildkit/util/progress/progressui"
 	"github.com/pkg/errors"
@@ -138,6 +139,13 @@ func runBuildBuildKit(dockerCli command.Cli, options buildOptions) error {
 		}
 		s.Allow(sp)
 	}
+	if len(options.ssh) > 0 {
+		sshp, err := parseSSHSpecs(options.ssh)
+		if err != nil {
+			return errors.Wrapf(err, "could not parse ssh: %v", options.ssh)
+		}
+		s.Allow(sshp)
+	}
 
 	eg, ctx := errgroup.WithContext(ctx)
 
@@ -408,3 +416,26 @@ func parseSecret(value string) (*secretsprovider.FileSource, error) {
 	}
 	return &fs, nil
 }
+
+func parseSSHSpecs(sl []string) (session.Attachable, error) {
+	configs := make([]sshprovider.AgentConfig, 0, len(sl))
+	for _, v := range sl {
+		c, err := parseSSH(v)
+		if err != nil {
+			return nil, err
+		}
+		configs = append(configs, *c)
+	}
+	return sshprovider.NewSSHAgentProvider(configs)
+}
+
+func parseSSH(value string) (*sshprovider.AgentConfig, error) {
+	parts := strings.SplitN(value, "=", 2)
+	cfg := sshprovider.AgentConfig{
+		ID: parts[0],
+	}
+	if len(parts) > 1 {
+		cfg.Paths = strings.Split(parts[1], ",")
+	}
+	return &cfg, nil
+}