Heads up when upgrading to node18 images: Dockerignore now works

[badrange]

This is not a bug, but a heads up you could write down in some upgrade guide:

After we merged PR where we upgraded to the new node 18 silta images we had an issue where sso didn't work. It turned out that an env var was missing, and after a bit of digging I realized that the missing env var used to live in nexts' .env.local file (it was generated in CircleCI).

A lot of head scratching later I realized that the .env.local file was missing because of a match in dockerignore.

So what has changed is that Dockerignore has started working, but it didn't before.