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builder: add note about alternative syntax 

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
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Sebastiaan van Stijn 2020-09-23 13:05:00 +02:00
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docs/reference/builder.md
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@ -1025,25 +1025,43 @@ The environment variables set using `ENV` will persist when a container is run
from the resulting image. You can view the values using `docker inspect`, and from the resulting image. You can view the values using `docker inspect`, and
change them using `docker run --env <key>=<value>`. change them using `docker run --env <key>=<value>`.
> **Note** Environment variable persistence can cause unexpected side effects. For example,
setting `ENV DEBIAN_FRONTEND=noninteractive` changes the behavior of `apt-get`,
and may confuse users of your image.
If an environment variable is only needed during build, and not in the final
image, consider setting a value for a single command instead:
```dockerfile
RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y ...
```
Or using [`ARG`](#arg), which is not persisted in the final image:
```dockerfile
ARG DEBIAN_FRONTEND=noninteractive
RUN apt-get update && apt-get install -y ...
```
> **Alternative syntax**
> >
> Environment variable persistence can cause unexpected side effects. For example, > The `ENV` instruction also allows an alternative syntax `ENV <key> <value>`,
> setting `ENV DEBIAN_FRONTEND=noninteractive` changes the behavior of `apt-get`, > omitting the `=`. For example:
> and may confuse users of your image.
>
> If an environment variable is only needed during build, and not in the final
> image, consider setting a value for a single command instead:
> >
> ```dockerfile > ```dockerfile
> RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y ... > ENV MY_VAR my-value
> ``` > ```
> >
> Or using [`ARG`](#arg), which is not persisted in the final image: > This syntax does not allow for multiple environment-variables to be set in a
> single `ENV` instruction, and can be confusing. For example, the following
> sets a single environment variable (`ONE`) with value `"TWO= THREE=world"`:
> >
> ```dockerfile > ```dockerfile
> ARG DEBIAN_FRONTEND=noninteractive > ENV ONE TWO= THREE=world
> RUN apt-get update && apt-get install -y ...
> ``` > ```
>
> The alternative syntax is supported for backward compatibility, but discouraged
> for the reasons outlined above, and may be removed in a future release.
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