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>

docs/reference/dagger/op.md

@@ -144,6 +144,16 @@ _No input._
 
 _No output._
 
+## op.#SaveImage
+
+### op.#SaveImage Inputs
+
+_No input._
+
+### op.#SaveImage Outputs
+
+_No output._
+
 ## op.#Subdir
 
 ### op.#Subdir Inputs

docs/reference/docker/README.md

@@ -40,6 +40,26 @@ A container image that can run any docker command
 
 _No output._
 
+## docker.#Load
+
+Load a docker image into a docker engine
+
+### docker.#Load Inputs
+
+| Name                | Type                                 | Description                                          |
+| -------------       |:-------------:                       |:-------------:                                       |
+|*tag*                | `string`                             |Name and optionally a tag in the 'name:tag' format    |
+|*source*             | `dagger.#Artifact`                   |Image source                                          |
+|*load.command*       | `"docker load -i /src/image.tar"`    |Command to execute                                    |
+|*load.registries*    | `[]`                                 |Image registries                                      |
+
+### docker.#Load Outputs
+
+| Name             | Type              | Description        |
+| -------------    |:-------------:    |:-------------:     |
+|*ref*             | `string`          |Image ref           |
+|*digest*          | `string`          |Image digest        |
+
 ## docker.#Pull
 
 Pull a docker container