Improve developer experience with Docker

Fix "Running as root is not recommended" message at container startup. See https://github.com/ocaml/opam/issues/2575.
Add Makefile targets to build and run Docker images, and 'make help' target.
Reduce number of layers in Dockerfile and improve build time.
Fix several bugs in Makefile which were causing spurious rebuilds.
Remove legacy extract.sh script.
Use HTTPS when downloading libmozjs.
Improve readability of Dockerfile and Makefile. Split out dependency installation into a separate Bash script.
Reduce verbosity of application and Docker builds.
Fix mistake in Cgo configuration for R, although R is still broken because using Cgo with libR requires Go 1.10.5 while Ubuntu Bionic only ships 1.10.4. See https://github.com/golang/go/issues/27496.

replit / prybar