Improve Docker build caching

[tcarreira]

Docker build will cache each layer with a temporary container and burst the entire layer cache if some change applies to that layer. This will have a great impact on build time for every update on package*.json.

We can leverage Docker Build Mounts and add a build caching mount and keep the npm cache between docker builds, even when cache is invalidated. This has a major improvement on developing the node application with docker.