docker socket forwarding support

- This PR adds a new mount type: `docker.sock` (in addition to `cache`
  and `tmp`)
- It's then able to mount the LOCAL (as in, from the machine running
  dagger) docker socket inside the container by pretending to be an SSH
  Agent (hijacking the SSH agent forwarding support of buildkit)

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

solver/dockersocketprovider.go

@@ -0,0 +1,62 @@
+package solver
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"time"
+
+	"github.com/moby/buildkit/session"
+	"github.com/moby/buildkit/session/sshforward"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/metadata"
+)
+
+const (
+	DockerSocketID   = "docker.sock"
+	DockerSocketPath = "/var/run/docker.sock"
+)
+
+type DockerSocketProvider struct {
+}
+
+func NewDockerSocketProvider() session.Attachable {
+	return &DockerSocketProvider{}
+}
+
+func (sp *DockerSocketProvider) Register(server *grpc.Server) {
+	sshforward.RegisterSSHServer(server, sp)
+}
+
+func (sp *DockerSocketProvider) CheckAgent(ctx context.Context, req *sshforward.CheckAgentRequest) (*sshforward.CheckAgentResponse, error) {
+	id := sshforward.DefaultID
+	if req.ID != "" {
+		id = req.ID
+	}
+	if id != DockerSocketID {
+		return &sshforward.CheckAgentResponse{}, fmt.Errorf("invalid socket forward key %s", id)
+	}
+	return &sshforward.CheckAgentResponse{}, nil
+}
+
+func (sp *DockerSocketProvider) ForwardAgent(stream sshforward.SSH_ForwardAgentServer) error {
+	id := sshforward.DefaultID
+
+	opts, _ := metadata.FromIncomingContext(stream.Context()) // if no metadata continue with empty object
+
+	if v, ok := opts[sshforward.KeySSHID]; ok && len(v) > 0 && v[0] != "" {
+		id = v[0]
+	}
+
+	if id != DockerSocketID {
+		return fmt.Errorf("invalid socket forward key %s", id)
+	}
+
+	conn, err := net.DialTimeout("unix", DockerSocketPath, time.Second)
+	if err != nil {
+		return fmt.Errorf("failed to connect to %s: %w", DockerSocketPath, err)
+	}
+	defer conn.Close()
+
+	return sshforward.Copy(context.TODO(), conn, stream, nil)
+}

solver/solver.go

@@ -157,7 +157,11 @@ func (s Solver) Export(ctx context.Context, st llb.State, img *dockerfile2llb.Im
 
 	opts := bk.SolveOpt{
 		Exports: []bk.ExportEntry{output},
-		Session: []session.Attachable{s.opts.Auth, s.opts.Secrets},
+		Session: []session.Attachable{
+			s.opts.Auth,
+			s.opts.Secrets,
+			NewDockerSocketProvider(),
+		},
 	}
 
 	ch := make(chan *bk.SolveStatus)