Switch the TypeScript template to a three-stage build

[fnesveda]

Just an experiment to speed up TypeScript actor builds using three-stage Docker builds:

• 1st stage from base image to install only production dependencies
• 2nd stage which starts from 1st stage, which also installs dev dependencies, copies source files, and runs the TypeScript build
• final stage from base image, which copies the prod dependencies from 1st stage and built folder from 2nd stage

It's better over the current two-stage builds because you don't have to install the production dependencies twice (once in each stage), and you don't have to explicitly prune the npm cache.

[fnesveda]

@mnmkng I did some very non-scientific testing and it felt faster, but some proper testing would be needed to see if it's actually better.

[fnesveda]

I don't really have the capacity to move forward with this right now, so I'm closing the PR, but @B4nan feel free to reopen and pick it up if it's something you guys would be interested in.