Add devcontainer config

[forabi]

Pull Request Template

Description

This should help make it easier to build and install the extension using VSCode (or any other devcontainer-supporting IDE). The gnome shell extensions dir is mounted directly at the corresponding location inside the container so the install command should work.

Checklist

• [x] My code follows the style guidelines of this project
• [x] My commits follow the commit standards of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have checked my code and corrected any misspellings

[Totto16]

I improved a few things, that would be too complicated to tell you, so I just did them myself:

general:

• move Dockerfile to not clutter the root dir
• add suggested extensions to customizations
• add postCreateCommand to install packages after create

Dockerfile:

• use better image in Dockerfile
• collapse multiple RUN commands into one, to reduce image layers
• install yarn in Dockerfile, otherwise the user will be prompted about that later

I used a newer image in the Dockerfile, since there is no reason to use a specific 20.x.y version, we just want to use the latest 20.x.y version and also there is no reason not using the latest alpine image

I also tested this locally and it seems to work as expected with these changes.

I'll wait until merging this, so that you can say, if something I did is not the best for a devcontainer. From my side it's approved.

And thank you for your contributiion ❤️