Recommend Docker image in getting started instructions

[rachitnigam]

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Our CI now uses the docker image. We should make it the recommended way to work with Calyx. Couple of complexities I ran into when setting up CI that we should talk about in the instructions:

1. PYTHONENV needs to contain flit's install location. See the Dockerfile's last few lines to get the exact ENV command needed.
2. fud check might fail if the user running the image is not root. Recommend people copy /root/.config/* into ~/.config/ so that they get the fud configuration

[rachitnigam]

Note that the Calyx and Dahlia repos need to be updated when using the containers

[anshumanmohan]

~Requesting that the Docker start off with turnt installed! Requesting because #1482 adds testing for MrXL.~ update: not needed; will use Runt instead