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>

tests/compute.bats

@@ -65,11 +65,11 @@ setup() {
     run "$DAGGER" compute --input-string 'in=foobar' "$TESTDIR"/compute/input/default
     assert_success
     assert_line '{"in":"foobar","test":"received: foobar"}'
-    
+
     run "$DAGGER" compute --input-string=foobar "$TESTDIR"/compute/input/default
     assert_failure
     assert_output --partial 'failed to parse input: input-string'
-    
+
     run "$DAGGER" compute --input-dir=foobar "$TESTDIR"/compute/input/default
     assert_failure
     assert_output --partial 'failed to parse input: input-dir'
@@ -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"]
+	},
+]