Dockerignore: Ignore everything except a file and the Dockerfile

If you need to ignore everything except some directories or files and also ignore some unnecessary files inside those allowed directories you can use the following .dockerignore file:

# Ignore everything
*

# Allow files and directories
!/file.txt
!/src

# Ignore unnecessary files inside allowed directories
# This should go after the allowed directories
**/*~
**/*.log
**/.DS_Store
**/Thumbs.db