This PR adds a dockerfile for use with Gitpod.
This provides all the dependencies, and kicks off a build once the
editor is opened for the first time.
It can be tested by going to
https://gitpod.io/#https://github.com/leanprover/lean4/pull/6382
This should make it less painful for users hoping to contribute small
lemmas to `Init/` and `Std/`; they can open gitpod and wait, rather than
having to read the docs to run a series of commands.
