Infrastructure and build optimizations

[btobin]

• Optimizes build stages in Dockerfile, makes test stage an entrypoint
• Cleans up Makefile, adds new utility commands
• Removes unnecessary env vars and var files
• Upgrades check-links.sh script, makes this easy to run in Docker container and as part of build
• Removes null-safety tests
• Removes unnecessary NPM packages
• Updates action build file, removing unnecessary installs and simplifying flow
• Updates README

@khanhnwin Take a look please and we can discuss!