.dockerignore file
Before the docker CLI sends the context to the docker daemon, it looks for a file named .dockerignore in the root directory of the context. If this file exists, the CLI modifies the context to exclude files and directories that match patterns in it. This helps to avoid unnecessarily sending large or sensitive files and directories to the daemon and potentially adding them to images using ADD or COPY.
The CLI interprets the .dockerignore file as a newline-separated list of patterns similar to the file globs of Unix shells. For the purposes of matching, the root of the context is considered to be both the working and the root directory. 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. Neither excludes anything else.
If a line in .dockerignore file starts with # in column 1, then this line is considered as a comment and is ignored before interpreted by the CLI.
Here is an example .dockerignore file:
# comment
*/temp*
*/*/temp*
temp?
This file causes the following build behavior:
| Rule | Behavior |
|---|---|
# comment |
Ignored. |
*/temp* |
Exclude files and directories whose names start with temp in any immediate subdirectory of the root. For example, the plain file /somedir/temporary.txt is excluded, as is the directory /somedir/temp. |
*/*/temp* |
Exclude files and directories starting with temp from any subdirectory that is two levels below the root. For example, /somedir/subdir/temporary.txt is excluded. |
temp? |
Exclude files and directories in the root directory whose names are a one-character extension of temp. For example, /tempa and /tempb are excluded. |
Matching is done using Go’s filepath.Match rules. A preprocessing step removes leading and trailing whitespace and eliminates . and .. elements using Go’s filepath.Clean. Lines that are blank after preprocessing are ignored.
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.
Lines starting with ! (exclamation mark) can be used to make exceptions to exclusions. The following is an example .dockerignore file that uses this mechanism:
*.md
!README.md
All markdown files except README.md are excluded from the context.
The placement of ! exception rules influences the behavior: the last line of the .dockerignore that matches a particular file determines whether it is included or excluded. Consider the following example:
*.md
!README*.md
README-secret.md
No markdown files are included in the context except README files other than README-secret.md.
Now consider this example:
*.md
README-secret.md
!README*.md
All of the README files are included. The middle line has no effect because !README*.md matches README-secret.md and comes last.
You can even use the .dockerignore file to exclude the Dockerfile and .dockerignore files. These files are still sent to the daemon because it needs them to do its job. But the ADD and COPY commands do not copy them to the image.
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.
Note: For historical reasons, the pattern . is ignored.
