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.

CHANGELOG.md

@@ -0,0 +1,15 @@
+## Unreleased
+
+### Fixed
+
+- Fixed the default value for `ACTIONS_CACHE_URL` when using GitHub action caching. (https://github.com/pulumi/pulumi-docker-build/pull/80)
+
+## 0.0.2 (2024-04-25)
+
+### Fixed
+
+- Upgraded pulumi-go-provider to fix a panic during cancellation.
+
+## 0.0.1 (2024-04-23)
+
+Initial release.