implement .daggerignore

Signed-off-by: Tony Worm <tony@hofstadter.io>

tests/ignore/main.cue

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"dagger.io/dagger"
+	"dagger.io/llb"
+)
+
+dir: dagger.#Artifact
+
+ignore: {
+	string
+	#compute: [
+		llb.#FetchContainer & { ref: "debian:buster" },
+		llb.#Exec & {
+			args: ["bash", "-c", "ls -lh /src > /out.txt"]
+			mount: "/src": { from: dir }
+		},
+		llb.#Export & { source: "/out.txt" },
+	]
+}
+