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build.md: Document --build-arg without value 

This use case is currently _working correctly_, which is nice, but there is no documentation to be found about it. This PR fixes that.

Signed-off-by: Per Lundberg <perlun@gmail.com>
This commit is contained in:
Per Lundberg 2018-04-12 11:58:13 +03:00
parent 3d0fc8d309
commit 82c23208ee
1 changed files with 14 additions and 2 deletions
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docs/reference/commandline/build.md
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@ -425,6 +425,18 @@ Dockerfile are echoed during the build process.
For detailed information on using `ARG` and `ENV` instructions, see the For detailed information on using `ARG` and `ENV` instructions, see the
[Dockerfile reference](../builder.md). [Dockerfile reference](../builder.md).
You may also use the `--build-arg` flag without a value, in which case the value
from the local environment will be propagated into the Docker container being
built:
```bash
$ export HTTP_PROXY=http://10.20.30.2:1234
$ docker build --build-arg HTTP_PROXY .
```
This is similar to how `docker run -e` works. Refer to the [`docker run` documentation](https://docs.docker.com/engine/reference/commandline/run/#set-environment-variables--e---env---env-file)
for more information.
### Optional security options (--security-opt) ### Optional security options (--security-opt)
This flag is only supported on a daemon running on Windows, and only supports This flag is only supported on a daemon running on Windows, and only supports