Many tools use ignore files to exclude files from build, process or publish steps (e.g. .npmignore
for npm, .gitignore
for Git and .dockerignore
for Docker). As your project grows, evolves, it can be hard to maintain the exclusion patterns and, as a side effect, you can expose unwanted or sensitive files. Using your .dockerignore
as a whitelist can be an elegant option to avoid these mistakes and to reduce the effort to keep things up to date.
What's the .dockerignore
for?
When you're creating a Docker image, the very first step is to define the build instructions in a Dockerfile
. The second step is to run the docker build PATH
command from the Docker CLI. The PATH
argument is the path to the folder containing your Dockerfile
. As a result, the Docker CLI will send the build context
to the Docker daemon.
The build context
Every file and folder within the given PATH
are tar
d by the Docker CLI and sent to the Docker daemon. These files and folders compose the build context
. Your Docker image will be built with this context
, so unwanted files sent to the daemon can be packaged by error in the final image with the ADD
or COPY
instructions.
Also, the Docker daemon can be executed on a remote machine, so it's better to avoid sending large, unnecessary and sensitive files to it.
Remember, ADD
and COPY
instructions are run by the daemon and never by the CLI, so they are not used to remove unwanted files from the build context
.
Exclude files from the build context
When you invoke the docker build
command, the Docker CLI looks for a file named .dockerignore
in the root folder of the build context
(i.e. in the root folder of the given PATH
). If this file exists, every file and folder matching the exclusion patterns will not be included in the tar
archive sent to the Docker daemon. The Docker build will be faster and the risk of packaging unwanted files reduced.
For example, if you want to exclude the .git
folder and all markdown files from the build context
, add the following content to your .dockerignore
:
.git
*.md
Use your .dockerignore
as a whitelist
Blacklist vs whitelist is a long debate and, I think, both types have its use case. From my experience, you'll have more files to exclude from your build context
than to include, so to keep things clean and secure, whitelisting files is less error-prone.
If you've read the .dockerignore
documentation, you may have noticed the following lines:
Finally, you may want to specify which files to include in the context, rather than which to exclude. To achieve this, specify * as the first pattern, followed by one or more! exception patterns.
Pretty easy!
As an example, the following .dockerignore
instructs the Docker CLI to exclude everything excepted the JS
files within the lib
folder from the build context
:
# Ignore everything
*
# Allow files and folders with a pattern starting with !
!lib/**/*.js
With a whitelist it's also possible to publish unwanted or sensitive files from a whitelisted folder but, in my opinion, this issue is minor.
Summary
Now that you know how to blacklist or whitelist files with your .dockerignore
, choose the best option for your use case. To conclude, using your .dockerignore
as a whitelist allows to:
- Exclude by default everything from the build
context
- Reduce the size of the
tar
archive sent to the Docker daemon - Reduce the risk of packaging unwanted or sensitive files in your Docker
image
- Reduce the effort to keep exclusion patterns up to date
PS: You can use the same trick with your .gitignore
or .npmignore
. For npm, the best practice is to use the files property in your package.json
instead of a .npmignore
file.
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