netlify: enable image customization; improve tests

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

pkg/universe.dagger.io/netlify/test/testutils/testutils.cue

@@ -0,0 +1,26 @@
+package testutils
+
+import (
+	"universe.dagger.io/bash"
+	"universe.dagger.io/alpine"
+)
+
+// Assert the text contents available at a URL
+#AssertURL: {
+	url:      string
+	contents: string
+
+	run: bash.#Run & {
+		input: image.output
+		script: "contents": """
+			test "$(curl \(url))" = "\(contents)"
+			"""
+	}
+
+	image: alpine.#Build & {
+		packages: {
+			bash: {}
+			curl: {}
+		}
+	}
+}