Add devcontainer, linters and formatters

[rlaiola]

Reference issue

None.

What does this implement/fix?

This PR aims at taking advantage of the Visual Studio Code Dev Containers extension to develop inside a Docker container. A development container is a running container with a well-defined tool/runtime stack and its prerequisites. The setup of this Dev container includes relevant linters and code formatters, as well as all dependencies necessary to automatically build and run RelaX on launch.

How to test this PR?

• Install and configure Docker Desktop for your operating system;
• Install VS Code and the Dev Containers extension;
• Clone this repository and open it in VS Code;
• Use the Dev Containers: Reopen in Container command from the Command Palette (Ctrl+Shit+P Linux/Windows or Cmd+Shit+P on OS X);
• When VS Code restarts, all dependencies will be installed automatically in the dev container. After that you can open a browser window and access the webapp at http://localhost:8088/;
• Close the window to stop the container;