Simplify docker.#Run

* `cmd` is renamed to `command` for readability
  * `script` is removed. Feature moves up the stack (bash.#Run,
    python.#Run)

Signed-off-by: Solomon Hykes <solomon@dagger.io>

pkg/universe.dagger.io/docker/test/nested-build-test.cue

@@ -14,12 +14,12 @@ dagger.#Plan & {
 						source: "alpine"
 					},
 					docker.#Run & {
-						cmd: name: "ls"
+						command: name: "ls"
 					},
 				]
 			},
 			docker.#Run & {
-				cmd: name: "ls"
+				command: name: "ls"
 			},
 		]
 	}