Add Dockerfile for development container

[cknitt]

See #6851 - people were asking for a dev container.

This adds a Dockerfile and a Makefile entry for building a container with all the required dependencies installed.
I am not familiar with the Development Containers spec as such and leaving that to someone else.

Example usage:

make dev-container
docker run --rm -it -v `pwd`:/data -w /data rescript-dev-container bash
opam exec -- make
opam exec -- make lib
opam exec -- make test
# etc.