Improve Development Accesibility - Devcontainer [part 1 / 2]
## Whats new 
This PR introduces some changes that improve the development experience via VSCode's devcontainers. Specifically:
* It removes the cloning of the pandas repo inside the container (it assumes that the user already cloned it on the host) and instead leverages the fact the the code is mounted to `/workspaces/pandas`.
* Defines the conda env inside the workspace itself such that it can be reused even when the container is deleted. This enables you to use the codebase outside the repo afterwards if you activate the same local conda env.
* Splits the previous `Dockerfile` in two: a simplified `Dockerfile` + a new `container_create.sh` that only runs the first time the container starts, making it possible to install the dependencies inside the workspace.

In the next PR I want to add support for Gitpod which is a nice way to develop when you don't have access to your PC. cc @fangchenli @datapythonista @WillAyd  This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Closing this, since there is no agreement on moving forward, and it didn't have any progress for couple of months. Hey @jreback and @datapythonista!
Sorry for the haitus, I got some time again and will be trying to move this forward.