Support loading artifacts into a Docker Engine

This adds support to loading artifacts (e.g. docker.#Build,
os.#Container, ...) into any arbitrary docker engine (through a
dagger.#Stream for UNIX sockets or SSH for a remote engine)

Implementation:
- Add op.#SaveImage which serializes an artifact into an arbitrary path
  (docker tarball format)
- Add docker.#Load which uses op.#SaveImage to serialize to disk and
  executes `docker load` to load it back

Caveats: Because we're doing this in userspace rather than letting
dagger itself load the image, the performance is pretty bad.

The buildkit API is meant for streaming (get a stream of a docker image
pipe it into docker load). Because of userspace, we have to load the
entire docker image into memory, then serialize it in a single WriteFile
LLB operation.

Example:

```cue
package main

import (
	"alpha.dagger.io/dagger"
	"alpha.dagger.io/docker"
)

source: dagger.#Input & dagger.#Artifact

dockersocket: dagger.#Input & dagger.#Stream

build: docker.#Build & {
	"source": source
}

load: docker.#Load & {
	source: build
	tag:    "testimage"
	socket: dockersocket
}
```

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

stdlib/dagger/op/op.cue

@@ -81,6 +81,12 @@ package op
 	ref: string
 }
 
+#SaveImage: {
+	do:   "save-image"
+	tag:  string
+	dest: string
+}
+
 #FetchGit: {
 	do:          "fetch-git"
 	remote:      string