fix(per-commit) add a nightly label

Historically, kong-build-tools would update the `latest` tag
for each new per-commit release. Since acfda16, that is no longer the
case. Given it's useful to have a generic name for the latest build of
kong - and we want to avoid pushing a tag named `latest`, so as to avoid
surprises when users pull `kong/kong`- this commit adds a `nightly` tag.
Such tag will be used, for example, in decK's CI, where we want to test
against the latest available build of Kong.

release-kong.sh

@@ -74,6 +74,11 @@ function push_docker_images() {
     -a "$DOCKER_REPOSITORY:$KONG_VERSION-$RESTY_IMAGE_BASE" \
        "$DOCKER_REPOSITORY:$ARCHITECTURE-$KONG_VERSION"
   docker manifest push "$DOCKER_REPOSITORY:$KONG_VERSION-$RESTY_IMAGE_BASE"
+
+  docker manifest create \
+    -a "$DOCKER_REPOSITORY:nightly" \
+       "$DOCKER_REPOSITORY:$ARCHITECTURE-$KONG_VERSION"
+  docker manifest push "$DOCKER_REPOSITORY:nightly"
 }