Add .devcontainer setup

[jdrueckert]

Visual Studio Code allows for local containerized development environments that avoid high setup and maintenance overheads.
With GitPod we already use something similar, yet not local.

The idea is to create a devcontainer configuration similar to the one we currently use for our website but customized for the AdventureSite.
The Dockerfile should ideally use a minimal (alpine) base image for node applications to accelerate loading times while providing all necessary dependencies.
This Dockerfile can then also be referenced in the gitpod.yml to avoid double maintenance.