{do:"subdir"} to select a subdirectory in a script

Signed-off-by: Solomon Hykes <sh.github.6811@hykes.org>

tests/subdir/simple/main.cue

@@ -0,0 +1,29 @@
+package main
+
+hello: {
+	string
+
+	#dagger: compute: [
+		{
+			do:  "fetch-container"
+			ref: "alpine"
+		},
+		{
+			do: "exec"
+			args: ["mkdir", "-p", "/tmp/foo"]
+		},
+		{
+			do: "exec"
+			args: ["sh", "-c", "echo -n world > /tmp/foo/hello"]
+		},
+		{
+			do:  "subdir"
+			dir: "/tmp/foo"
+		},
+		{
+			do:     "export"
+			source: "/hello"
+			format: "string"
+		},
+	]
+}