Various fixes (#63)

* Fix digest resolution and attestation style

* Add a bunch more tests

* Rename fields for consistency

* Remove copy-pasta

* Value -> pointer

test/testdata/local-policy/mapping.yaml

@@ -0,0 +1,29 @@
+# map repos to policies
+version: v1
+kind: policy-mapping
+policies:
+  - origin:
+      domain: docker.io
+      prefix: library/
+    id: test-images
+    description: Local test images
+    files:
+      - path: "doi/policy.rego"
+
+mirrors:
+  - policy-id: test-images
+    mirror:
+      domains: ["*"]
+      prefix: "repo"
+  - policy-id: test-images
+    mirror:
+      domains: ["*"]
+      prefix: "library/"
+  - policy-id: test-images
+    mirror:
+      domains: ["*"]
+      prefix: "test-image"
+  - policy-id: test-images
+    mirror:
+      domains: ["*"]
+      prefix: "image-signer-verifier-test"