Create Dockerfile by following google-cloud-go's internal/librariangen/Dockerfile.

We will probably use sdk-platform-java. The folder name requires a bit of thought.

• Maybe the name "librariangen" also works for Java.
• The "internal" seems the convention of google-cloud-go repository.
• sdk-platform-java already has "hermetic_build" folder in the root.

This decision is easy to reverse later. We only need to update the AR Exit Gates flow definition.