Update docker action to v2 in CI workflows

At the moment we are using the v1 version of the action that builds and pushes the docker images used by our Github Actions CI.

The goal is to migrate to version v2, since v1 was deprecated.

Workflows to be updated:

• https://github.com/nuxeo/nuxeo-web-ui/blob/maintenance-3.0.x/.github/workflows/preview.yaml#L77 (Web UI)
• https://github.com/nuxeo/nuxeo-web-ui-gh-runner/blob/master/.github/workflows/build.yaml#L26 (GH Actions Runner)

Migration guide: https://github.com/docker/build-push-action#upgrade-from-v1