Fix coala's cache when `--user` is defined

Docker has this weird way of using `root` as the default user resulting in files which the user can't write to. The only way to fix this is to declare `--user=` but this'll cause a problem because there's no home folder so it defaults to `/`. The workaround for this is to pre-made `/.local/share/coala` and set the permission to `777` (RWX for everyone)

Start by inspecting the Docker image setup and how coala's cache path is configured for a numeric --user. Check the image behavior with and without --user=; done means the cache is writable without relying on a user's home directory or leaving the container unable to start.

Written by the indexing model from the issue text.