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/run-export-directory-test.cue

@@ -13,10 +13,13 @@ dagger.#Plan & {
 
 		run: docker.#Run & {
 			"image": image.output
-			script: #"""
-				mkdir -p test
-				echo -n hello world >> /test/output.txt
-				"""#
+			command: {
+				name: "sh"
+				flags: "-c": #"""
+					mkdir -p test
+					echo -n hello world >> /test/output.txt
+					"""#
+			}
 			export: {
 				directories: "/test":      _
 				files: "/test/output.txt": _ & {