Add a developer container configuration

[JamesHutchison]

A developer container allows you to set-up up a reproducible environment for devs. It allows them to, quite literally, jump onto a project and start coding, writing and running tests, linting, and formatting correctly in a couple minutes or less.

An existing configuration was already created for this project, which can be found at this closed PR:

https://github.com/pydantic/pydantic-extra-types/pull/27

[dmontagu]

@JamesHutchison I would suggest creating this issue on the https://github.com/pydantic/pydantic repo, which will get a lot more visibility

[JamesHutchison]

Created https://github.com/pydantic/pydantic/issues/5437

[yezz123]

Close this in favor of this one #27