Add devcontainer in support for codespaces

[StijnKas]

This one supersedes #130 - adding devcontainer with proper configuration makes 1) codespaces work, 2) docker creation seamless and 3) environments consistent.

Building the environment takes quite a while, because we don't just install a pretty sizable python image but also install all app dependencies & quarto. Let's see if we can improve this after.

[StijnKas]

@operdeck @yusufuyanik1 could you give the new codespaces thing a shot? Should be as simple as just hitting the code button on the homepage and then clicking Create codespace on master. Be sure to kill your codespace after using it on https://github.com/codespaces