Using .dockerignore Files to Exclude Files from the Build Context

`COPY` can't refer to local files: ```console $ ls main.c $ docker build -<<< $'FROM scratch\nCOPY main.c .' [+] Building 0.0s (4/4) FINISHED => [internal] load build definition from Dockerfile 0.0s => => transferring dockerfile: 64B 0.0s => [internal] load .dockerignore 0.0s => => transferring context: 2B 0.0s => [internal] load build context 0.0s => => transferring context: 2B 0.0s => ERROR [1/1] COPY main.c . 0.0s ------ > [1/1] COPY main.c .: ------ Dockerfile:2 -------------------- 1 | FROM scratch 2 | >>> COPY main.c . 3 | -------------------- ERROR: failed to solve: failed to compute cache key: failed to calculate checksum of ref 7ab2bb61-0c28-432e-abf5-a4c3440bc6b6::4lgfpdf54n5uqxnv9v6ymg7ih: "/main.c": not found ``` ## .dockerignore files You can use a `.dockerignore` file to exclude files or directories from the build context. ```text # .dockerignore node_modules bar ``` This helps avoid sending unwanted files and directories to the builder, improving build speed, especially when using a remote builder. ### Filename and location When you run a build command, the build client looks for a file named `.dockerignore` in the root directory of the context. If this file exists, the files and directories that match patterns in the files are removed from the build context before it's sent to the builder. If you use multiple Dockerfiles, you can use different ignore-files for each Dockerfile. You do so using a special naming convention for the ignore-files. Place your ignore-file in the same directory as the Dockerfile, and prefix the ignore-file with the name of the Dockerfile, as shown in the following example. ```text . ├── index.ts ├── src/ ├── docker │ ├── build.Dockerfile │ ├── build.Dockerfile.dockerignore │ ├── lint.Dockerfile │ ├── lint.Dockerfile.dockerignore │ ├── test.Dockerfile │ └── test.Dockerfile.dockerignore ├── package.json └── package-lock.json ``` A Dockerfile-specific ignore-file takes precedence over the `.dockerignore` file at the root of the build context if both exist. ### Syntax The `.dockerignore` file is a newline-separated list of patterns similar to the file globs of Unix shells. Leading and trailing slashes in ignore patterns are disregarded. The following patterns all exclude a file or directory named `bar` in the subdirectory `foo` under the root of the build context: - `/foo/bar/` - `/foo/bar` - `foo/bar/` - `foo/bar` 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. ```gitignore #/this/is/a/comment ``` If you're interested in learning the precise details of the `.dockerignore` pattern matching logic, check out the [moby/patternmatcher repository](https://github.com/moby/patternmatcher/tree/main/ignorefile) on GitHub, which contains the source code. #### Matching The following code snippet shows an example `.dockerignore` file. ```text # 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`. |

This section explains how to use `.dockerignore` files to exclude files and directories from the Docker build context. It covers the filename and location of the `.dockerignore` file, including how to use Dockerfile-specific ignore files. It also describes the syntax of the `.dockerignore` file, including comments and pattern matching rules. A link to the moby/patternmatcher repository is provided for more detailed information on pattern matching logic.