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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
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,
> 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:
> The `ENV` instruction also allows an alternative syntax `ENV <key> <value>`,
> omitting the `=`. For example:
>
> ```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
> ARG DEBIAN_FRONTEND=noninteractive
> RUN apt-get update && apt-get install -y ...
> ENV ONE TWO= THREE=world
> ```
>
> The alternative syntax is supported for backward compatibility, but discouraged
> for the reasons outlined above, and may be removed in a future release.
## ADD