universe: python: allow custom path to python

Closes #2114

Signed-off-by: Adam Smith <adamsmith@nottheeconomist.com>

pkg/universe.dagger.io/python/python.cue

@@ -42,7 +42,7 @@ import (
 
 	docker.#Run & {
 		command: {
-			name:   "python3"
+			name:   string | *"python3"
 			"args": ["\(_mountpoint)/\(script._filename)"] + args
 		}
 

pkg/universe.dagger.io/python/test/test.cue

@@ -3,13 +3,28 @@ package python
 import (
 	"dagger.io/dagger"
 	"dagger.io/dagger/core"
-
+	"universe.dagger.io/docker"
 	"universe.dagger.io/python"
 )
 
 dagger.#Plan & {
 	actions: test: {
 
+		// Run with a custom path to python
+		customPath: {
+
+			// python:3.10-alpine has both python3 and python in the PATH
+			_image: docker.#Pull & {
+				source: "python:3.10-alpine"
+			}
+			run: python.#Run & {
+				input: _image.output
+				command: name: "python"
+				script: contents: #"print("Hello, world!")"#
+			}
+			// This needs no output test because it is only testing that the command runs
+		}
+
 		// Run a script from source directory + filename
 		runFile: {