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>

client/client.go

@@ -114,7 +114,11 @@ func (c *Client) buildfn(ctx context.Context, st *state.State, env *environment.
 	// Setup solve options
 	opts := bk.SolveOpt{
 		LocalDirs: localdirs,
-		Session:   []session.Attachable{auth, secrets},
+		Session: []session.Attachable{
+			auth,
+			secrets,
+			solver.NewDockerSocketProvider(),
+		},
 	}
 
 	// Call buildkit solver

environment/pipeline.go

@@ -468,6 +468,11 @@ func (p *Pipeline) mount(ctx context.Context, dest string, mnt *compiler.Value)
 				llb.Scratch(),
 				llb.Tmpfs(),
 			), nil
+		case "docker.sock":
+			return llb.AddSSHSocket(
+				llb.SSHID(solver.DockerSocketID),
+				llb.SSHSocketTarget(dest),
+			), nil
 		default:
 			return nil, fmt.Errorf("invalid mount source: %q", s)
 		}

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)

stdlib/dagger/op/op.cue

@@ -52,7 +52,7 @@ package op
 	// `true` means also ignoring the mount cache volumes
 	always?: true | *false
 	dir:     string | *"/"
-	mount: [string]: "tmpfs" | "cache" | {from: _, path: string | *"/"} | {secret: _}
+	mount: [string]: "tmpfs" | "cache" | "docker.sock" | {from: _, path: string | *"/"} | {secret: _}
 	// Map of hostnames to ip
 	hosts?: [string]: string
 	// User to exec with (if left empty, will default to the set user in the image)

tests/compute.bats

@@ -106,6 +106,10 @@ setup() {
     assert_output "secret=mySecret"
 }
 
+@test "compute: docker socket" {
+    run "$DAGGER" compute  "$TESTDIR"/compute/dockersocket
+}
+
 @test "compute: exclude" {
     "$DAGGER" up -w "$TESTDIR"/compute/exclude
 }

tests/compute/dockersocket/main.cue

@@ -0,0 +1,18 @@
+package main
+
+import (
+	"dagger.io/dagger/op"
+	"dagger.io/docker"
+)
+
+TestDockerSocket: #up: [
+	op.#Load & {
+		from: docker.#Client
+	},
+
+	op.#Exec & {
+		always: true
+		mount: "/var/run/docker.sock": "docker.sock"
+		args: ["docker", "info"]
+	},
+]