The .dockerignore file is similar to the .gitignore syntax. Here are some example rules:

# Ignore a file or directory in the context root named "modules"
modules

# Ignore any files or directories within the subdirectory named "modules" 
# in the context root
modules/*

# Ignore any files or directories in the context root beginning with "modules"
modules*

# Ignore any files or directories one level down from the context root named
# "modules"
*/modules

# Ignore any files or directories at any level, including the context root, 
# named modules
**/modules

# Ignore every file in the entire build context (see next rule for how this 
# could be used)
*

# Re-include the file or directory named "src" that may have been previously
# excluded. Note that you cannot re-include files in subdirectories that have 
# been previously excluded at a higher level
!src

Note that "build context" is the directory you pass at the end of your build command, typically a . to indicate the current directory. This directory is packaged from the docker client, excluding any files you have ignored with .dockerignore, and sent to the docker daemon to perform the build. Even when the daemon is on the same host as your client, the build only works from this context and not directly from the folders.

There is only a single .dockerignore for a build, and it must be in the root of the build context. It will not work if it is in your home directory (assuming you build from a subdirectory), and it will not work from a subdirectory of your build context.

To test what is in your current build context and verify your .dockerignore file is behaving correctly, you can copy/paste the following (this assumes you do not have an image named test-context, it will be overwritten and then deleted if you do):

# create an image that includes the entire build context
docker build -t test-context -f - . <<EOF
FROM busybox
COPY . /context
WORKDIR /context
CMD find .
EOF

# run the image which executes the find command
docker container run --rm test-context

# cleanup the built image
docker image rm test-context

You may have noticed that if you put a Dockerfile in your home, and launch a docker build you will see a message uploading context, this means docker creates a .tar with all the files in your home and in all the subdirectories, and uploads this tar to the docker daemon.

If you have some huge files, this may take a long time.

So to avoid this you may

1) create a specific directory, where you put your Dockerfile, and all what is needed for your build

2) tell docker to ignore some files during the build

So you may put in the .dockerignore all the files not needed for your build

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