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incorporate doc review comments

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Signed-off-by: Madhav Puri <madhav.puri@gmail.com>
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Madhav Puri
2017-06-02 00:07:09 +00:00
committed by Tibor Vass
parent 40a2dac738
commit 9fab23902f
3 changed files with 32 additions and 37 deletions
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docs/reference/builder.md
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@@ -1058,19 +1058,20 @@ useful interactions between `ARG` and `ENV` instructions:
4 RUN echo $CONT_IMG_VER
```
The command line passes the `--build-arg` and sets the `v2.0.1` value. And the `ARG
CONT_IMG_VER` is defined on line 2 of the Dockerfile. On line 3, the `ENV`
instruction of the same name resolves to `v2.0.1` as the build-time variable
was passed from the command line and expanded here.
Unlike an `ARG` instruction, `ENV` values are always persisted in the built
image. Consider a docker build without the --build-arg flag:
```
$ docker build Dockerfile
```
Using this Dockerfile example, `CONT_IMG_VER` is still persisted in the image but
its value would be `v1.0.0` as it is the default set in line 3 by the `ENV` instruction.
The variable expansion technique in this example allows you to pass arguments
from the command line and persist them in the final image by leveraging the `ENV`
instruction. Variable expansion is only supported for the `Dockerfile` instructions
described [here](#environment-replacement).
Unlike an `ARG` instruction, `ENV` values are always persisted in the built image. If
`docker build` were run without setting the `--build-arg` flag, then
`CONT_IMG_VER` is still persisted in the image but its value would be `v1.0.0`.
from the command line and persist them in the final image by leveraging the
`ENV` instruction. Variable expansion is only supported for [a limited set of
Dockerfile instructions.](#environment-replacement)
Docker has a set of predefined `ARG` variables that you can use without a
corresponding `ARG` instruction in the Dockerfile.