Use Actions git context instead of dynamically created buildkit one (#25381)

The [docker/build-push-action@v2
action](https://github.com/docker/build-push-action) by default ignores
the checkout created using the actions/checkout@v2 action. When you pass
a git build context to docker build, it wouldn't include the .git
directory.

By passing `context: .` to the build step then it'll use the Actions git
context which includes the git fetch from the earlier step.

.github/workflows/release-nightly.yml

@@ -78,12 +78,14 @@ jobs:
       - name: build rootful docker image
         uses: docker/build-push-action@v4
         with:
+          context: .
           platforms: linux/amd64,linux/arm64
           push: true
           tags: gitea/gitea:${{ steps.clean_name.outputs.branch }}
       - name: build rootless docker image
         uses: docker/build-push-action@v4
         with:
+          context: .
           platforms: linux/amd64,linux/arm64
           push: true
           file: Dockerfile.rootless