🤖 Update `.dockerignore` file

[ErikSchierboom]

To help both speedup Docker builds and prevent new containers being created when unrelated files changes, this PR adds some rules to the .dockerignore file (or add the file when it didn't exist). See https://docs.docker.com/engine/reference/builder/#dockerignore-file for more information on .dockerignore files and what they do.

Tracking issue

See https://github.com/exercism/exercism/issues/6113

[andrerfcsantos]

I can't merge this as it is waiting an approval from @exercism/maintainers-admin - feel free to merge.