Docker ignores patterns in .dockerignore

Question

I intended to exclude python generated files from the docker image. So I added .dockerignore at the root of the context directory:

# Ignore generated files *.pyc 

Alas docker build ignores this and copies the entire directory tree that looks like the following:

/contextdir/ |-- Dockerfile \-- src/     |-- a.py   # this is copies - all right     \-- a.pyc  # this should be ignored, but is copied too. Why? 

Answer 1

Finally, I understood what's the trick. For some reason, I wasn't able to find any mention of this and haven't found any example Dockerfile with such construct, so documenting it here. Was it trivial for everybody else?

The reference at How to create a dockerignore file doesn't put clear enough that patterns like *.pyc are matched only at the beginning of the path, or for the files directly below the context directory, but not recursively. To make it work recursively the **/ syntax must be used:

# Ignore generated files **/*.pyc 

Answer 2

The docs for .dockerignore state that the pattern matching use's Go's filepath matching logic. The docs also cover the recursive pattern:

Beyond Go’s filepath.Match rules, Docker also supports a special wildcard string ** that matches any number of directories (including zero). For example, **/*.go will exclude all files that end with .go that are found in all directories, including the root of the build context.