Switch docker cache to github actions backend

We were failing to push cached images to the registry, because we're no
longer logging to the GHCR. In addition, secrets.GITHUB_TOKEN would need
to be expanded to support writing to the container registry, and giving
that to arbitrary PRs isn't great.

The GitHub Actions build cache is auto-clearing after 7 days, and
requires no auth.

Author: anoadragon453

.github/workflows/docker.yaml

@@ -65,5 +65,5 @@ jobs:
           labels: "gitsha1=${{ github.sha }}"
           tags: "${{ steps.set-tag.outputs.tags }}"
           platforms: linux/amd64,linux/arm64
-          cache-from: type=registry,ref=ghcr.io/${{ github.repository }}:buildcache
-          cache-to: type=registry,ref=ghcr.io/${{ github.repository }}:buildcache,mode=max
+          cache-from: type=gha,
+          cache-to: type=gha,mode=max