Using https://github.com/devcontainers/features/tree/main/src/conda, and similar to what we currently have for gitpod.

Preliminary but working version: https://github.com/tobiasdiez/sage/tree/conda-devcontainer

CC: @isuruf @dimpase @saraedum @mkoeppe @slel

Component: build

Issue created by migration from https://trac.sagemath.org/ticket/34560