Fix GitHub action URL (#80)

Our docs and default value for the GitHub action cache URL were
referring to the
[wrong](https://github.com/tonistiigi/go-actions-cache/blob/master/api.md)
environment variable.

I was under the impression that the underlying library ends up checking
for the correct variable later so this would be only cosmetic, but a
user
[confirmed](https://github.com/pulumi/pulumi-docker-build/issues/77#issuecomment-2135621983)
this does appear to break things.

I brought over an old test I had written while this was still in the
pulumi-docker repo, but unfortunately it doesn't currently test GHA due
to ci-mgmt limitations
https://github.com/pulumi/pulumi-docker-build/issues/82.

Fixes https://github.com/pulumi/pulumi-docker-build/issues/77.

examples/tests/caching/index.ts

@@ -0,0 +1,40 @@
+import * as buildx from "@pulumi/docker-build";
+import * as pulumi from "@pulumi/pulumi";
+
+const config = new pulumi.Config();
+
+const start = new Date().getTime();
+
+// docker buildx build \
+//  -f Dockerfile \
+//  --cache-to type=local,dest=tmp,mode=max,oci-mediatypes=true \
+//  --cache-from type=local,src=tmp \
+//  --build-arg SLEEP-MS=$SLEEP_MS \
+//  -t not-pushed \
+//  -f Dockerfile \
+//  .
+const img = new buildx.Image(`buildx-${config.require("name")}`, {
+  tags: ["not-pushed"],
+  dockerfile: { location: "Dockerfile" },
+  push: false,
+  context: { location: "." },
+  buildArgs: {
+    SLEEP_SECONDS: config.require("SLEEP_SECONDS"),
+  },
+  cacheTo: [{ raw: config.require("cacheTo") }],
+  cacheFrom: [{ raw: config.require("cacheFrom") }],
+  // Set registry auth if it was provided.
+  registries: config.require("username")
+    ? [
+        {
+          address: config.getSecret("address"),
+          username: config.getSecret("username"),
+          password: config.getSecret("password"),
+        },
+      ]
+    : undefined,
+});
+
+export const durationSeconds = img.ref.apply(
+  (_) => (new Date().getTime() - start) / 1000.0
+);