Several cases:

• exceptions to exclusions that contain wildcards do not work: !README*.md;
• special wildcard string ** that matches any number of directories (including zero) is not supported;
• directory exclusions do not work, however https://docs.docker.com/engine/reference/builder/#dockerignore-file states:

For example, the patterns /foo/bar and foo/bar both exclude a file or directory named bar in the foo subdirectory of PATH or in the root of the git repository located at URL